MPEG means Motion Picture Expert Group and it's THE resource for video formats in general.
This group defines standards in digital video, among it the MPEG1 standard (used in Video CDs), the MPEG2 standard (used on DVDs and SVCDs), the MPEG4 standard and several audio standards - among them MP3 and AAC.
Files containing MPEG-1 or MPEG-2 video often use either the .mpg or .mpeg extension.