UMD is an acronym for Universal Media Disc, which was created by Sony for its Playstation Portable console. A UMD is basically a miniature version of a DVD which is capable of holding 1.8 GB of information. Sony uses these discs to store not only game information for the console, but for popular movies as well.
If you've watched television recently, you've likely seen at least one commercial for the upcoming retail release of a movie, publicizing that the film would be available on DVD and UMD for PSP.
Thankfully for PSP owners, it's not just popular movies that have been released in the UMD format. A wide variety of older films are available, along with season sets of television shows, documentaries, and music-related titles as well. With the popularity of the PSP system continuing to grow, the catalog of UMD video titles released for the Playstation Portable is sure to grow in the coming years.