IPSW

Originally standing for "iPod Software," IPSW files are the core of the iDevice update/restore system. While existing since the second generation iPod Classic, the internal structure is different.

Format

IPSW files, at least for iDevices, are simply renamed ZIP files. As such, they can be opened using any ZIP program (some may require the extension to be changed to .zip first).

IPSW files follow a very structured format. The root filesystem, ramdisks, kernelcache, and manifests are in the root directory, while everything else is underneath the Firmware folder.

Example Newer IPSW

Here is the internal structure of the IPSW for the 12.0 (build 16A366) build for the iPhone XR (iPhone11,8):

Example Older IPSW

Here is the internal structure of the IPSW for the 2.0 (build 5A347) build for the original iPhone (iPhone1,1). Note how fewer files there are: