DAT is employed to refer to a certain tape backup format. But also in audio/video terminology it normally means files that VideoCD has in its SEGMENT or MPEGAV directiories. These DAT files are basically MPEG-1 files by having an additional information and certain specific file structure -- they are NOT "real" MPEG-1 files and you also need to convert them back to "real" MPEG-1 files as a way to edit them even that a lot of with the software players treat them as regular MPEG-1 files.