This is the official Apple firmware update for iPod, iPhone and iPad.
It's a good idea to update your firmware for many reasons such as to add new features, improve mobile performance, battery interface, iOS etc. The iPod, iPhone and iPad Firmware is in IPSW format and can be applied via iTunes, through the restore option. The process is very easy and safe.
This particular version is for the iPhone 4 standard (not the S model). If the IPSW linked here does not work, we recommend you use the excellent utility ipswDownloader, which automatically downloads the IPSW you need.
Whether you need to undo a jailbreak or update the iPhone, iPod or iPad to a later version, this firmware is a must download.