What is in an IPSW file?
The IPSW file type is primarily associated with 'iPhone/iPod' by Apple Inc.. This is a firmware update file for the iPhone. The file is a renamed .ZIP file. If you unZIP it you will see a file called MANIFEST.PLIST and a file without an extension but with a version number in the name, e.g., Firmware-14.5.3. This file is the firmware image.

Source http://filext.com/file-extension/IPSW