ISO
From PHWiki
An ISO image, commonly called an ISO, is a file containing a disk image of a CD, DVD, or other optical media. The ISO shorthand and .iso file extension comes from the ISO 9660 file system specification published by ISO (International Organization for Standardization), the first file system used to store data on a CD.
An ISO image is a direct representation of the data on optical media, and can be 'burned' onto writable or rewritable CDs or DVDs. In piracy, ISO images of game discs are commonly shared on P2P networks to be downloaded by other users. Most games consoles will not play burned discs without modification.
PSP ISOs are a special case. Although UMD discs are ISO 9660 compliant, they are a proprietary size and therefore cannot be burned to standard 12cm DVDRs. However, using an ISO loader or custom firmware they can be played directly from the PSP's Memory Stick
