[HTC One X+] Ghand0ur:
I don't know all the answers to your questions but let me help you make something clear.

Unlocking the bootloader is different from "rooting"

Your warranty is voided when you unlock the bootloader, which is an important step to actually "root". Your bootloader is what you see when you reboot and hold volume down. The step of unlocking the bootloader is the step that cause you to lose your data. So I recommend you to connect your phone and copy all the valuable information and files on it to your PC, then once this step is done you can copy it back.

Rooting: Becoming the administrator of your phone (like sudo in linux) , Becoming a SUPERUSER , meaning you can change system files and so forth and so on (Thats the general explanation however since we all have S-ON so its called temporary root but lets not get ourselves to that yet)

How can you gain root access?

1-Unlock the bootloader

2-Install a custom recovery

3-flash "superuser.zip"(which will be included in almost all the guides that you will follow to root your phone)

Once you have root access you will be able to play with your phone settings , but be careful not to break the settings.

Now concerning your final question , how to "unroot"?

Before you flash "superuser.zip (which will be the real "rooting" of your phone) , you will go to your "custom recovery" (most popular onces are TWRP and CWM) and go to an option called "Backup" and you do a backup called "nandroid backup" , which copies your phone AS IT IS , which then you can copy it to your PC. You will be able to restore it using the recovery you had installed as it is. Which technically will "unroot" your phone.

Question you might ask: That is so cool, but can't I do it before unlocking the bootloader and losing all my data? from my EXPERIENCE and what I searched , No.

Then how will I benefit? Since having root access to your phone might put your phone in danger of breaking something , or installing a bad ROM (read about this topic before rooting. Here is a summary: "HTC Sense" is a ROM. "Samsung TouchWIZ" is a ROM, "Stock Android" is a ROM) you will have a full safe backup that you will be able to restore so you can have your phone running again.

Also note the following: Once you have a custom recovery you will be able to install custom roms , when you install custom roms your APPs are deleted BUT the data IN your SDcard directory will not be harmed (Music, pics..etc)

So how can I recover my applications? By installing "titanium backup" you will be able to backup all your applications apk and data, saved to your internal storage, so when you switch roms , you need to redownload the app and then restore your data, and you will have all your data back.

So why can't I use that before I unlock the bootloader? Because Titanium backup ITSELF requires root access to access your apps apks and do full backups.

Also a note, when flashing your FIRST ROM, you will sweat like a pig when you get stuck on "Boot screen" , its called a "BOOT LOOP" , do not fear. Since HTC has all the bootloaders S-ON (till now) you will need when you switch ROMS to flash the boot.img to your phone. HOW?

Google "Fastboot flash boot boot.img HTC" and you will see tons of guides. It should also be noted in every single custom ROM guide that you will need to follow that STEP.

ALSO!!! Do NOT FORGET TO MAKE A NANDROID BACKUP BEFORE ROOTING AND BEFORE ATTEMPTING TO FLASH A CUSTOM ROM!! AND DONT INSTALL A CUSTOM ROM FOR "AT&T" AND YOUR PHONE IS INTERNATIONAL AND VICE VERSA.

I rooted my phone and I wish I had someone to tell me that in a single post , so I spent around 10 minutes writing this guide , which I hope people will find useful.

Best of luck mate :)

Also note that this is the wrong section, since its a question it should go to "Q&A section".