CWM

ClockworkMod Recovery (CWM) is a replacement recovery option for Android devices, made by Koushik "Koush" Dutta. It is based on the Android 2.1 (Eclair) recovery image. Features includeNandroid backup, adb shell, advanced update.zip options (ignore asserts and signature checks), and file browser for choosing update.zips

The Rom Manager app may be used for installing CWM, overwriting the device's stock recovery. As with the stock recovery, CWM can apply software changes to the device. Unlike the stock recovery, CWM is able to entirely replace the device's primary Android operating system ("flashing a new ROM"), including with aftermarket replacements such as CyanogenMod. Once installed, CWM it is accessed by turning your phone off, then triggering the bootloader prompt by holding down some device-dependent button combination as it is turned on.

CWM is also known as clockwork and CW recovery.

Source : XDA