NOOBS or (New Out Of Box Software) is released by he Raspberry Pi Foundation. It incorprates a recovery system that loads every time the Raspberry Pi is booted. This recovery system contains pre-loaded images for 6 operating systems that can be installed on the device.
The operating systems include: Arch Linux, OpenELEC (Xbox Media Centre), Pidora, RISC OS, RaspBMC (Xbox Media Centre), Raspbian (Raspberry Pi Foundation’s preferred operating system).
With NOOBS, the first time you turn on the Pi a choice of operating systems automatically pops up on your display and you simply pick one and the Raspberry Pi does the rest.
Should the worst happen and you manage to corrupt your operating system you do not have to go through the process of downloading the operating system again and re-flashing your SD card, you can simply load up the recovery system and install a fresh operating system directly from the Raspberry Pi. To load the recovery system simply hold the SHIFT key down while you boot your Raspberry Pi.
The operating system images use around 1GB of space on the SD card and the rest of the card can be used to store your own files.