What is an ISO?

An ISO is a homebrew game file. These files have the extension ".iso".

If your PSP is running custom firmware (ex. 4.01m33-2) then you can use programs such as UMD Dumper to backup your PSP games to your memory stick as ISO files.

You can also find ISO files on the Internet for download. You will need a USB cable to get the files onto your PSP though. If you know your way around torrent files and have a torrent program then just start searching for your favorite PSP games.

For beginners looking for ISO files I recommend PSP Blender. Unlike other PSP download sites they charge a one time access fee versus monthly fees. After paying the one time fee you can access their database of games, movies and music for as long as you want and download as much as you want. They also offer great support and tutorials for downloading and moving the files to your PSP.