[HTC Rezound] dg4prez:
There seems to be a lot of confusion about what things are or how they work together so I'm gonna try and build a post to explain some things. If i get any of this wrong, feel free to call me out and I'll correct it. I can generally be relied upon to make mistakes.

First let's start with the various things that you're gonna be hearing about or messing with on your phone.

Hboot or bootloader

This is pretty much the lowest level thing you're gonna mess with. This is what makes the phone boot up, and where you can flash all sorts of things like new hboots, kernels, and whole roms depending on how they are packaged. You can get here by either powering off your phone then powering on with volume down + power, or by selecting "reboot to bootloader" in most custom roms. If you've entered your hboot from a powered off state, you'll start in hboot, also known as the bootloader. The bootloader is where you will load ph98img.zip files, factory reset, boot to recovery, or enter fastboot. Fastboot is the section of the bootloader that will allow you to run fastboot commands from your PC/mac/whatever to load things like boot images, recovery images, return your phone to an s-on state, etc. It's a very powerful interface. From here you have menu options which will allow you to reboot, power off, or return to the bootloader.

If you've entered hboot from the adb reboot bootloader command or your custom rom, you will start in fastboot, and can enter the bootloader from here.

When entering the bootloader using either method, the first thing the phone will do is look for a PH98IMG.zip file. If it finds one, it will want to install it. You'll have the option of installing it, or rebooting. That's it. This is why you want to get rid of the ph98img.zip files from the root of your sd card once you've used them - if you pooch your rom, you're not getting into recovery or fastboot until you remove that file. If you can't boot your phone, you better hope you have another SD card or a micro sd card reader kicking around somewhere. You can boot into fastboot by powering off the phone, then powering it on by holding power and volume down keys at the same time.

A word of warning - this is one of the few places you can actually "brick" your phone. Don't mess around with installing hboots unless you are absolutely certain that you a) have a need and b) have an md5 checksum of the file and have verified that checksum on your own gear. Generally speaking, the only time you need to flash the hboot is when you are upgrading in some way, for example when going from GB to ICS.

Quick note - when people refer to the "SD card patch" they are referring to a patch that allows you to use the GB firmware with ICS roms. This is largely unneeded at this time, since the main purpose was for folks who wanted to run ICS roms without upgrading their hboot - there's no going backward with hboot unless you have s-off.

Radio files

this is how your phone works with the radio. you can brick your phone here, too, so always check md5 before installing, and ask yourself if you really need to be installing it. new radio versions are generally given the credit for things like increased signal strength, faster 3g/4g, better power consumption, etc. these are often paired with a kernel, as well.

Recovery

This is a software layer that you can flash to your phone that allows you to boot the phone into a utility state where you can install roms, flash things like patches or modifications, backup your rom/kernel, etc. When people refer to a nandroid backup, they are referring to a backup made here. It's a snapshot of your phone that lets you mess with things and restore back to that point in time should you screw something up. You pretty much can't brick your phone at this level - it's all just files and filesystems. You can also mount your SD card to reader mode for connection to a computer, do a factory reset of data, and a few other fun things like root your rom here. You should only root your rom if it is a stock rom that has not been rooted yet. Rooting a rooted rom will usually unroot your root so you don't have root.

There are a couple of different recoveries. The rezound community seems to use primarily amon-ra, which is what i use, but there is also clockwork recovery as well. Recovery is usually installed by booting your phone into fastboot and running the "fastboot flash recovery <recovery file>" command. Recovery can also be installed via the flash_image binary found in the RomManager package (ex: flash_image recovery amonra.img). This eliminates the need for fastboot altogether. Just need a terminal, root, and the flash_image binary.

Kernel

This is basically the primary component of an OS. The kernel is where a lot of the functionality of the phone are made or broken. the setting that prevents use of setcpu to change the way the CPU is governed is here. The thing that prevents bluetooth from working right on sense 4 roms? Kernel. Everyone is waiting eagerly for the ICS kernel source to be released so custom kernels can be made that resolve these issues and more. You can flash a kernel via recovery if you've done s-off to your phone, or via a ph98img.zip file in the bootloader. With s-off, kernels can also be backed up in recovery. The kernel is often given much of the credit or blame for the performance of your phone, overheating, etc.

Roms

Your phone's OS, sometimes incorrectly referred to as firmware. It's not - the firmware is the stuff we've already covered that can brick your phone. Think of your phone more as a tiny computer, with the rom being the distribution that you're using. This is pretty apt since it's basically linux, and just like linux, the actual OS is just files and partitions. Because the radio interface layer requires sense, the bulk of our roms are all based on sense roms, and all the functional ones are. Developers modify them to add features, remove bloat or sense components, or take sense roms from other phones and adapt them for our own. You install a rom through recovery.

Never claim to have "bricked" your phone when installing a rom. You can't, and doing so only points out that you don't really know how your phone works. This is embarrassing and best avoided when possible so jerks like me do not tell you what a n00b you are, and to go read threads like this one. Since you're here, we can assume that you've bothered to research things before asking stupid questions, so bravo! In all seriousness, you can no more brick your phone by messing up the rom than you'd brick your PC by screwing up your windows install. You can always start again from the beginning and reinstall. You might lose apps and settings, but this is the nature of the beast. Always nandroid before messing about, and you'll be fine. Nandroid is covered further later on.

RUUs

Strictly speaking, the things that we refer to as RUUs aren't. Everyone calls them that anyway, and I don't see it going away anytime soon. Technically, however, an RUU is HTC's Rom Updater Utility, and it's an executable that gets run on your PC, not something you load to SD card and flash in your bootloader. That said, RUU has come to mean factory signed software/firmware packages that are released by the manufacturer or leaked. They come in the form of a ph98img.zip file that can install pretty much anything - hboot, radio, kernel, rom, etc. What is installed varies from package to package.

Installing an ruu can only be accomplished on a phone that is currently not in an htc dev unlocked state, or a phone that has s-off. when installing one of these that includes an hboot, you'll need to reinstall recovery as well - installing a new hboot via RUU will always require this as the custom recovery is overwritten with the stock recovery.

If your phone has not yet had the s-off procedure done and you intend to do so, make sure the RUU you are about to install does not break the s-off exploit. Doing so will lock you into that RUU until the dev updates the exploit to work with the new hboot (if included in the RUU) or another RUU is released.

HTCdev

This is the method HTC gives us to unlock our phones. it allows us to install recoveries and roms, but not a lot else. you cannot downgrade your hboot or radio files with this, for example. you can install kernels via ph98img.zip file, but not through recovery. basically, it's HTC's way of letting us mess with our phones but without (hopefully) giving us enough control to brick them. this is considered better than nothing, but much less than ideal since you still don't have total control over what your phone does and how. doing this does essentially void your warranty, though to date there have been no confirmed reports of this affecting a return via either warranty or insurance.

With your phone in a stock state, you can apply only factory signed RUUs. Roms and kernels are closed to you, and you can move only forward with your radio and hboot, never backward.

With your phone in an HTCdev unlocked state, you can apply unsigned RUUs that contain a boot, system, and recovery image - all components must be present.

To get your phone into this state, visit HTCdev.com.

S-Off

This sets your security flag on the phone to off.Stock, your phone comes with this flag set to on. Our s-off method does it at the radio layer, and at this time is thought to be irreversible by update. It isn't, though - it's just unlikely because there are legitimate s-off phones out there. Still, it's always a good idea to wait to see if that shiny new RUU is gonna make your phone boring again before you install it. To be safe, wait for an RUU that has the hboot removed and just applies kernel, radio, and system files. You can turn it back on with fastboot should you wish, however. Htcdev unlocking, a bit of wire or a paperclip, and a decent set of timing is required for this. In my opinion it is best to do this once you are sure your phone does not have problems that require a warranty return, but it should be done before upgrading to new, untested OTAs/RUUs. There's always the chance that HTC will block the exploit that gives us this, at which point you're stuck until the devs figure out another way or someone comes up with some crazy method like the evo 3d guys. Many are nervous about this process, and for good reason. While the devs have done everything they can to make this bulletproof, it is best to enter into this mod with as much knowledge as possible, just like any other mod. It's also best to be relaxed, have an internet connection other than your phone, and not have anywhere you have to be right away in case things go like you don't expect. Should things go wrong, don't freak out. Ask questions. Many "bricked" phones have been restored with simple application of knowledge.

Things s-off does: Allows you to flash kernels from recovery, allows you to flash any hboot and radio you want. Lets you install any "ruu" you want with any combination of boot image, system, and recovery, rather than the entire package. Makes rom installation easier on devs due to kernel installation in recovery.

Many have scoffed at s-off, claiming it does nothing that unlocking doesn't do already. This isn't really the case, as we've all seen how important the ability to downgrade firmware and radios can be. Personally, I think of a phone with s-on in much the same way I think of eating steak through a trash bag. If you have the option not to, what the hell are you doing?

Things s-off does not do: magically turn your phone into an open platform to which devs will instantly flock to give you the aosp roms you believe yourself entitled to. What will do that? Glad you asked.

RIL - radio interface layer

Basically, the RIL is a driver for the radio - it lets the radio interface with the software running on the phone. This is a new problem that effects 4g phones. It sucks. Basically, your manufacturers (also verizon, but you can assume they don't want you to do anything other than pay them.) don't want you touching their precious radio chipset with your grubby little paws. We don't have source code, and they're not talking. Of course we already have software on our phone that has the RIL stuff we need, right? Yes... sort of. For any sense roms we want to use. This is why we don't have aosp. For aosp, one of three things needs to happen:

1) Manufacturers grow a soul and give us source in the form of a leak or they go mad and just release it. This is pretty much as likely as Verizon deciding that they are doing away with early termination fees.

2) Someone reverse engineers our RIL and makes their own RIL software. This is as likely as my wife developing new roms for her Incredible 2. The one she blames for her school's ****ty email server and wants to trade in for an iphone.

3) Someone develops an interim layer that translates aosp to sense and back again. This was done on the thunderbolt, but it was wonky. Certain individuals are confident that this will be easier done on ICS and are working on it. Some AOSP action would be pretty damn awesome, but don't expect it, demand it, or hold your breath for it. It's hard, and is going to require a very intense combination of skill, knowledge, and tenacity.

Now i know what you're thinking - the Nexus is a CDMA 4g phone and they can do whatever the hell they want. Do they have an open RIL? The answer is no. They have an aosp (Android Open Source Project) RIL though, which amounts to basically the same thing. If it is any consolation they can't have sense roms as a result. Those poor bastards, right? This is why many are torn between the Nexus and the Rezound. The Nexus is open and a dev's paradise - CM9, AOKP, MIUI, ****ty roms that some kid slapped a godawful theme on - it's all there. The Rezound has better radios, better screen, and better build quality (subjective) but no aosp. If you're reading this you've already chosen or had the choice made for you by verizon's $50 sale.

Fast Boot

But Derek, you say, you already talked about fastboot in the bootloader section. We get it. Nay nay. This is fast boot. Note the space. Basically, HTC roms include a tech that puts the phone into a hibernation state rather than powering off. You'll find it in the power menu. When this box is checked, your phone will go from powered off to your home screen in a very short period of time - 15 seconds on my phone, vs the normal 45 second boot time. If you pull the battery, you will have a normal boot time. Personally, I could take or leave this feature. I almost never shut my phone off, and when I do, I want it to actually shut off because i'm shutting it down because I need a cold boot. It's up to you how useful this feature is to you.

Perflock, CPU Frequency, and Governers

Many people have had poor experiences running the ICS leaks due to the CPU maxing out. The stock HTC kernel uses perflock, which constantly resets the frequency and scaling of your CPU. This means that the use of programs like setcpu or system tuner to change the min/max frequencies or governer of your CPU will cause it to essentially run in a state where it is either using the max frequency, or the minimum, without scaling up and down according to need. Some CPU monitoring apps will cause a similar reaction. The best way to avoid all this unpleasantness and get the best battery life out of your phone is to simply leave it be and let it do it's thing, because quite frankly you can't stop it anyway. If you absolutely must see what your CPU is doing, I have had good results with CPU Monitor by Coconuts. I still don't recommend keeping this active full time as I suspect it still impacts performance and battery life, but many do without any obvious adverse results. As a side note, the HTC kernel uses the On Demand governer, which steps up the CPU frequency quickly when needed and scales it back down slowly. Clearly, HTC's focus was on performance rather than battery life, here. Once kernel source for ICS is released, devs can build custom kernels which do not include perflock, and add other nice features as well like full power usb charging. Once this occurs, we will be free to use setcpu to scale our CPUs up or down to our hearts content and use whatever governer we like. Until then the best advice is simply to leave it alone.

I will be updating this and adding to it constantly. I'm not an expert, just an average user who is trying to put some useful info in the same place. Please shoot me a PM with suggestions/things I got wrong and I will update this post and credit you.

Contributors who have helped make this more accurate

scotty1223, esheesle