UMD stands for "Universal Media Disc." It is Sony's proprietary format and is used for games and movies intended for play on the PSP (PlayStation Portable) handheld console.
Presently, UMDs -- even UMD movies -- can only be played on a PSP and it is unlikely that Sony will ever release a separate UMD player. Sony does not make a UMD burner for consumer use (PSP owners can instead use Memory Sticks for saving their own content).
Also Known As: Universal Media Disc
Universal Media Disk
Movies are often released in both DVD and UMD formats.