Darth Vader is the central antagonist in George Lucas' original Star Wars trilogy and his final prequel, Star Wars Episode III: Revenge of the Sith. In the original Star Wars trilogy, Vader is embodied by David Prowse, though Sebastian Shaw makes a brief cameo as the unmasked Vader. In Star Wars Episode III: Revenge of the Sith, Vader is played by Hayden Christensen. Every scene in which the character appears masked, James Earl Jones provides Vader's voice.

Vader is one of the most iconic villains in film history, and was listed as the third greatest movie villain on the American Film Institute list "AFI's 100 Years... 100 Heroes and Villains".

In the original trilogy, George Lucas depicts Darth Vader as a fearsome cyborg who acts as the supreme commander of the brutal Galactic Empire. Throughout the films, Vader oppresses the galaxy and hunts down the members of the Rebel Alliance in the service of his master, Emperor Palpatine. In Star Wars Episode VI: Return of the Jedi, the series' final chronological installment, Vader redeems himself by killing Palpatine and saving his son Luke (Mark Hamill), sacrificing himself in the process.

In the prequel trilogy, Lucas shows Vader as his former self, Anakin Skywalker, a slave boy who eventually becomes a Jedi Knight, and later, a hero in the Clone Wars. Palpatine manipulates Anakin into betraying the Jedi, and Anakin falls to the "dark side" of the mystical Force.

Reference: http://en.wikipedia.org/wiki/Darth_Vader