Darth Vader was a primary antagonist in the original Star Wars trilogy. He was a Sith Lord, wielding the Dark Side of the Force and a crimson lightsaber.
Born as Anakin Skywalker, he was a valiant, and extremely well-known Jedi Knight and hero of the Galactic Republic. When his mother was killed by Tusken Raiders young Skywalker went berserk, slaughtering an entire village of the desert people; the first signs of the dark side in the, then, protagonist. Approximately a week later, he committed an act that was both forbidden by the Jedi, and that led to his turn; he married the love-interest, Padme Amidala.
The Clone Wars had erupted and young Skywalker was exposed to the horrors of war. Towards the end of the wars, he received horrible dreams predicting the death of his wife, just as he had predicted the death of his mother. Desperately, he sought advice, knowledge to save his true love. On his question, then Chancellor Palpatine told him of a dark-side power that could save the life of his wife. Thinking over what his friend had said, he confronted Palpatine about his knowledge of the dark-side, revealing him to in-fact be Darth Sidious. Following a duel with several Jedi Masters, Skywalker took the final step to the dark-side, aiding in the murder of a prominent Jedi Councilmember, Mace Windu.
His first act as Darth Vader was to slaughter the "traitorous" Jedi in the Temple on Coruscant (capital of the Republic). After clearing the monument of his former brothers, Vader was ordered to the safe-house planet of the Separatist Council, leaders of the opposing forces, puppets of Sidious. Upon arriving, Vader slaughtered the Council, thinking he was their new guardian.
Soon, Amidala arrived, having been made aware of Vader's atrocities by Obi-Wan Kenobi, Skywalker's mentor. When Kenobi revealed himself to have been hidden aboard Amidala's ship, Vader assaulted his wife with a force-choke, rendering her unconscious. Kenobi and Vader dueled as Mentor attempted to bring his student back to the light. Failing through persuasion, Kenobi gained the high-ground and incapacitated Vader by removing his remaining left arm (his right had been taken in battle during the wars) and both legs. Lava from the planet on which they dueled burned what was left.
His body discovered by newly announced Emperor Palpatine, Vader received high-technology prosthetic limbs, lungs, and a full-body suit. This suit gave him the signature rasp, deep breathing and voice, and incredible strength and resilience.
Darth Vader was one of the first characters in Star Wars and the first antagonist. He gained a reputation in-universe as a ruthless killing machine, sword of the Emperor.
He has gained great fame among fans as a forceful, powerful character. Vader, as either himself, or as Skywalker, is present is many Star Wars novels.