MP3+G is not a real standard, but its become a "de facto" standard for karaoke files when played with computers. Essentially, MP3+G is a combination of two files -- one is a regular MP3 audio track, while the other one is a CDG file, typically ripped from a CD+G disc. The "+G" part provides the graphical content for the playback, which in most cases means the lyrics of the song.
Virtually all computer-based karaoke playback tools support MP3+G.
