.ipsw

The file format .ipsw is a software update for the Apple iPod line. These files are usually found in.

Inside it
The file format itself is a .zip but is renamed, so end-users don't get into it. There will be two files inside it. One will be called MANIFEST.PLIST and one called Firmware-some number here. The firmware file is the actual firmware for the device.

Programs using it
The iPhone unlock programs use this as well as iTunes itself