I know that there are a few things that you can flash.
1. CWM (Clock Work Mod) This is flashed on the recovery and once installed allows you to flash without a computer via your internal SD card.
2. Kernel This is what your phone uses to boot up and how to remove Carrier_IQ from the AT&T 2.35 build. Some Roms Overclock from here.
3. Rom This is where you can change what android version is installed as well as what apps are installed by default.
Tell me if I am correct in all this.