Tau was a trader whose people raided Voyager and stole its computer core and the Doctor's mobile emitter.
Tau was an alien dealer in stolen goods who operated from a planet where he bought and sold technology taken from passing ships. His raiders struck Voyager and made off with valuable equipment.
In Concerning Flight, Janeway set out to recover the ship's computer processor and the Doctor's mobile emitter, bringing along a holographic Leonardo da Vinci, who used the emitter to move about freely on the planet. Posing as traders, Janeway and Leonardo outwitted Tau and recovered the stolen technology.