A music demo, commonly called a songwriter demo, can be a simple guitar/vocal or a full band production. It can be produced at a home studio utilizing very basic equipment or recorded in a state-of-the-art facility.
Since the object of the recording is to sell the song the emphasis should be on the lyrics and melody. The vocalist usually ‘out front’ in the mix. The song demo is one of the few ways a songwriter can audition his/her songs to publishers, recording companies and internet listeners. Today, the music demo is more than likely to be in the mp3 format.
You don’t necessarily need a full band but most songs will benefit with at least four or five instruments. Adding extra instruments: background vocals, strings, etc. can greatly enhance the song and therefore your chance of placement.