During the rooting process, first thing you do is flash a recovery image. For instance with the droid 1, you may want to flash SP Recovery on the device and that will enable nandroid backups. Once rooted, have a ROM installed, and its stable and all, you may want to back it up to the SD card as a Nandroid backup. There's SP Recovery or ClockworkMod. ClockworkMod comes from Rom manager, an app from the market that allows you to download roms, kernels, themes, updates, perform backups, fix permissions, flash alternate recovery (SP Recovery), or partition the SD card. Rom manager makes things easier basically. There's the free version RM and the premium version at cost. That's my understanding.