IPSW files (iPod Software) are the files used by iTunes, Apple Configurator, and Xcode to restore or update an iOS device’s firmware. This includes the iOS operating system and the built-in apps. These files are compressed archives and can be downloaded manually from Apple’s iOS Dev Center, or automatically using iTunes or Apple Configurator. Once updated, there is no Apple-supported method for downgrading iOS on a device.
There is a different .ipsw file for each iOS version and device model. For example, an iPhone 5, iPad Mini Wi-Fi, iPad Mini Wi-Fi + Cellular, and a 4th Generation iPod Touch can all run iOS 6.1.2, but they will each use a different .ipsw file to install the iOS system software.
Looking at the name of an .ipsw file, we can learn all we need to know about its contents. For example, an ipsw file called…
…is for a 4th Generation iPod Touch, as indicated by “iPod4,1”, which is known as the model identifier for this product. After the first underscore, we find the commonly used version number of the iOS software contained within – in this case, 6.1.2. Next, we find the build number, or developers’ detailed version number – in this case, 10B146. After the final underscore, we see “Restore.ipsw” which is the common suffix for every .ipsw file.