DAT can mean lots of things -- often it is used to refer to a certain tape backup format. In a/v terminology it normally refers (at least most questions are focused on this one) to files that VideoCD has in its SEGMENT or MPEGAV directiories. These DAT files are basically MPEG-1 files with an additional information and certain specific file structure -- they are NOT "real" MPEG-1 files (their header data is slightly different when stored on VCD) and you need to convert them back to "real" MPEG-1 files in order to edit them even that most of the software players treat them as regular MPEG-1 files.
