Lets ask our good friends at Wikipedia:
Darth Vader (born Anakin Skywalker) is a central character in the Star Wars saga, appearing as the chief antagonist in the original trilogy and one of the main protagonists in the prequel films. He is first depicted as Darth Vader, a Dark Lord of the Sith in the first of the three original films. He is then revealed in Star Wars Episode V: The Empire Strikes Back and in the subsequent prequel films to be originally a Jedi Knight called Anakin Skywalker who falls to the dark side of the Force; he is also revealed to be the father of both Luke Skywalker and Princess Leia Organa, the two main protagonists of the original trilogy. Luke ultimately redeems his father in Star Wars Episode VI: Return of the Jedi, and Anakin sacrifices himself to save his son.