[aman1127]

Stickies on the first page will tell you everything:

Pasted from there:

******** Nand Backup and Nand Restore********
There are 2 options for making backups and restoring.
1) The easy method is to use Rom Manager. Install Rom Manager from the Market.
2) Put your phone into Recovery Mode and do all the steps manually.

Section 3 - Rom Manager Nand Backup/Restore -
3A) Go to the Market and download/install "Rom Manager" by ClockworkMod and then open it.
There is a free one and a donation one.

3B) Once it's opened, select "Flash ClockworkMod Recovery"

3C) Confirm your phone is an HTC Evo (CDMA)
Wait a couple minutes while it does it thing. When it's finished you'll get a "Successfully flashed ClockworkMod recovery!"

3D) Select "Backup Current Rom" and you'll have the option of naming it. Default is the Date and Time (2010-06-23-22.25.19) YYYY-MM-DD-HH-MM-SS
And that's it. Rom is backed up. If you try to flash another rom and don't like it or you make changes and mess something up, you can just restore the backup you just made and be right back where you were.

3E) To restore the Rom, select "Manager and Restore Backups"

3F) Select the Rom from the list of available backups. (You'll have the option to Restore/Rename/Delete)

3G) Choose Restore and select which parts you want to restore. All of them are checked by default.
You're choices are;
Restore Boot
Restore System
Restore Data
Restore Cache
Restore SD-Ext

Selecting all them will do a complete restore of your rom. I haven't experimented to see what the different parts do.