What is a "recovery mode"?
Android's recovery mode allows rom-flashing even if the device is currently unusable. Invoking it usually involves a key combination that is pressed when starting the device. These combinations differ from device to device, and might also depend on the recovery image used.
- clockworkmod: the most famous custom recovery
- twrp: Team Win Recovery Project is an alternative custom recovery
- backup: from the custom recoveries, one can create/restore Nandroid Backups
- nandroid: a backup type offered by ClockworkMod and other custom recoveries, but not by stock recoveries