Darth Vader is a Sith Lord and apprentice of Darth Sidious, aka Palpatine, or the Emperor. He used to be a Jedi called Anakin Skywalker but he turned evil when he realised its cooler. His master used to be Obi-Wan Kenobi, but he later kills him onboard the Death Star. Darth Vader used to look normal but in his first duel with his former master he was badly burned and lost all his limbs, so he became a cyborg to stay alive.
Vader betrayed his former Jedi friends and slaughtered thousands of Jedi in the Jedi Temple. (He even killed his own wife!) He also helped the Emperor establish the Galactic Empire and Assemble the massive army of Stormtroopers. Vader trained a Pseudo-Sith, Starkiller, to be his apprentice but was betrayed by him. Aww.
When he realises he has a son, Luke Skywalker, he began to doubt his evilness. This changed completely when he realised that he also had a daughter, Princess Leia, and he realised he was sick of Palpatine's bullshit and threw him down a chasm. The redeemed Skywalker then died in his son's arms.