|ClockWorkMod. Image courtesy: XDA Developers Forum|
Recovery is a bootable partition of Android operating system and where lot of repairing tools available. We can compare recovery with BIOS in computer. ClockworkMod recovery is an alternative recovery for stock Android. It can give lot of options rather than stock Android recovery.
Installation of CWM (ClockworkMod) recovery using other tools (e.g) ADB and Fast Boot tool) seems difficulty for new kids in Android arena. CWM allows backup and restore of stock ROM. Using CWM user can update/replace stock Android and install other custom ROM like Cyanogenmod.