[Galaxy S II Q&A, Help & Troubleshooting] xtachix:
dont worry about the csc means really dont worry, every official rom, custom rom, comes with csc. some of them have multiple csc.

CWM = clockworkmod (this is the recovery integrated with the kernel. you have a few main types- cwm, twrp, official). this recovery allows you to install roms and such. version 6 is the latest. if i'm not mistaken the version you have for gingerbread would be (if you have cwm) 4 or 5.

cm = cyanogenmod (this is the rom)

they are two different things.

let us go through the basics:

what does a ROM have?

it has a kernel (this allows you to do the basic io stuff, it has cwm or twrp >> if custom, or official recover if official)

it has a bootloader (this allows you to get into a specific version of roms), official roms have the bootloader

it has system files (this is the actual rom base, it has all the modules)

it has basic apps (well, these are the apps, dialer, messaging, etc..)

it has csc

it has baseband and ril

let us elaborate a little bit:

your phone currently has gingerbread bootloader. so it is version 2.0+

to use Icecream sandwich you need the icecream sandwich bootloader. 4.0+

different version means different bootloader

this is where flashing a rom via odin comes in handy. it will install that specific bootloader into your phone

ofcourse along with other things, like the kernel, actual rom, apps. so you will have a full 4.1.2 system.

notice: ics and jb bootloaders are the same, reason is simple, the base version of both is 4, if there they introduce a version 5 android then everyone has to get the newest bootloader for version 5 to use it.

it doesnt matter where the 4.x+ comes from, if it is aosp aokp official miui, the bootloader is REQUIRED to run any of those

now your first step is to install the official rom (this way you ensure that you have the proper bootloader, and at the same time you get the official 4.1.2). now if you want to keep the official then there is nothing else to do, if by any chance you need to change csc then the steps there are simple, but it is not required. look up what is csc

next you need to install a kernel that has either cwm or twrp (this way you prepare yourself to install custom roms)

next you select the rom you would like to install, download it and put it in your sdcard

next you boot into recover (notice it is different from the official one, it has many options)

install the rom from the sdcard location

and in many cases you will have to install gapps (google apps, and do not make a mistake and install the wrong one, every version of android has its own gapps, if you have 4.2.2 install ONLY 4.2.2 gapps)

done!

as i mentioned above, every rom comes with whatever that it comes with. a kernel, csc, blah blah blah.

so basically what happens when you flash the custom rom?

simple, you change the kernel (zImage) it has its own and that's what it needs to run, and official kernel cannot run an aosp rom. period.

you put a whole new framework, system, apps, based on the android version they have been developed in.

next if you want to modify things, you can get into themes, installing another kernel, changing baseband and ril(Radio Interface Layer), and so on.

well i hope this helps