Basically, once you root your handset and you want to try out a new rom you used to have to connect your phone to a pc and push the recovery files to be able to do anything.
Then cloclworkmod recovery was written that allowed the recovery files to be stored and started from the SD card, therefore removing the dependancy on a pc to push the files.
Now this new one, unrevoked, flashes the recovery files directly to the recovery section on your phone. No need for a PC and no dependancy on the files being on the SD card. So now, once you flash this, you simply reboot your phone with the volume button held down. Wait a few seconds then scroll ro 'recovery' with the volume button, then press the power button. This takes you directly to recovery menu that allows you to backup your current rom, flash new roms, wipe your cache etc.