What is ClockworkMod Recovery and what is it for?
ClockworkMod Recovery is a "custom recovery image" which is installable on many Android devices. You can install it in order to replace your device's "stock recovery image" with something more useful. Specifically, it can be used to backup and restore your current ROM and data, as well as to install new ROMs.
You can use ROM Manager to manage which ClockworkMod Recovery is installed (you need one specifically for your device), to easily boot into ClockworkMod Recovery, and to install ROMs.