A custom recovery is a recovery environment created by a third party. It can be used to replace the default, stock recovery environment with a customized recovery environment on the target device. Here is a list of the most common features that can be found in custom recovery:

• It provides full backup and restores functionality (such as NANDroid).
• It allow unsigned update packages or allows signed packages with custom keys.
• It selectively mounts device partitions and SD card.
• It provides USB mass storage access to SD card or data partitions.
• It provides full ADB access, with the ADB daemon running as root.
• There's a fully featured BusyBox binary. BusyBox is a collection of powerful command line tools in a single binary executable.

There are several custom recovery images available in the market today, such as ClockworkMod recovery, TeamWin Recovery Project, and so on. The following screenshot shows the options available with TeamWin Recovery Project (TWRP) 3.0.0.0: