They're ethereal creatures called notes (called that because they speak in music). They appear as fluctuating balls of light, but can go invisible at will on the material plane. They are attracted to a bard's mystical music.
EDIT: Oh, and a bard gets a new one every 5 levels, starting at first...so they end up with 5 notes at level 20. Their stats are nowhere near as cool as a familiar's though, and they have no real use in combat (can't deliver touch spells, etc...) but they can give the bard some groovy abilities, and after a while can be used to report on the look of things in the ethereal plane.