Data is a Lieutenant Commander on board the USS Enterprise, and is the closest thing the ship has to a Science Officer. He is an android and is therefore an invaluable source of knowledge and information, as well as a useful candidate for away teams due to his superior strength, resilience and mental capabilities. His immunity to things like bacterial or viral infections, radiation or gamma rays, most forms of primitive firearm and the effects of intense heat and cold make him a useful scout for dangerous missions, and he sometimes completes solo missions with help from the Enterprise.
Data has no genuine Human emotions, but he has learnt to approximate Human emotions over time which has led to a massive expansion of his neural net. This expansion of not just his knowledge and experience but also his personality have given him amazingly accurate humanlike qualities, to the point where some people wonder whether he is Human or not.