Darth Vader is the main villain of the original Star Wars movie trilogy and the Sith Lord alter ego of (spoiler alert) Anakin Skywalker. He is also the father of (more spoilers) Luke Skywalker and Princess Leia. Darth Vader is a pop-culture icon known for his awesome and evil mask as well as his red lightsaber and having the voice of James Earl Jones.
Darth Vader is redeemed (oh man, more spoilers) when he saves Luke Skywalker in Return of the Jedi and kills the Emperor, being mortally wounded himself in the process. His spirit appears with that of Obi-Wan Kenobi and Yoda in the final scene of the Star Wars film saga.