The PSP uses the Universal Media Disc (UMD) format for its games, but with the advent of custom firmware and homebrew, ISO and CSO (Compressed ISO) formats became popular alternatives. These formats allow for the distribution and playback of PSP games on custom firmware-enabled devices, often reducing file sizes and enabling faster loading times. The ISO format is an uncompressed image of the UMD, while CSO is a compressed version, making it more convenient for distribution.