Darth Vader is a cybernetically-augmented Human Sith Lord and military commander of the mighty empire that governs his home galaxy. He is second in command in the imperial military hierarchy, taking direct orders from Emperor Palpatine (a.k.a. Darth Sidious), who mentored him in the ways of the Force.
Vader was born on the desert world of Tatooine, and trained as a Jedi Knight by Master Obi-Wan Kenobi to serve the Galactic Republic. He was extremely skilled as a warrior, and had immense control over the Force. However, his power hunger and frustration with the Jedi Order led him to fall to the dark side and become a Sith, being mentored by Senator Palpatine. While fighting his former mentor Kenobi, Vader fell into a pit of lava and what remained of his body was put into a life-support armor, making him a cyborg.
When the Republic was overthrown and Palpatine became Emperor, Vader was made a commander and put in charge of the creation of the Empire's mightiest weapon, the Death Star, capable of destroying an entire planet as means of intimidation. However, technical plans that could reveal the Death Star's weakness were stolen by soldiers of the Rebel Alliance. The Emperor tasked Vader, along with Grand Moff Tarkin, to retrieve the stolen plans. To do so they successfully captured the ship transporting rebel agent Princess Leia Organa and held her prisoner, with Tarkin going as far as to order the destruction of the Princess's home planet, Alderaan. Unknown to them, however, the plans had been hidden in the droid R2-D2, who was sent to the planet Tatooine along with fellow droid C-3PO and met the young Luke Skywalker and an aged Obi-Wan Kenobi, now living in exile as a hermit. The four of them along with two smugglers named Han Solo and Chewbacca managed to infiltrate the Death Star and save the Princess. During the rescue, however, Vader dueled with and ultimately killed Kenobi.
While Luke Skywalker escaped and the plans allowed the Rebels to coordinate an attack to successfully destroy the Death Star, this wouldn't be the last confront between Luke and Darth Vader. After training with Master Yoda to become a Jedi, Luke fought Vader on the planet Bespin, where Vader revealed that he was actually Luke's father and asked him to join the dark side as he did, claiming this was Luke's destiny. The third and final confrontation between them happened aboard the second Death Star which was being constructed in orbit of the Forest Moon of Endor. Despite Luke defeating him and the Emperor telling him to kill Vader and join the dark side to replace him, Luke refused and preferred to spare his father's life. The Emperor tortured Luke as a result, but in a final act of redemption, Vader killed the Emperor and took off his own life support helmet in order to die peacefully.
He was last seen as a ghost in Endor, along with the ghosts of Obi-Wan Kenobi and Master Yoda.
As a leader, Vader is ruthless, ambitious and thoroughly despotic. He has little tolerance for failure and brutally punishes those who disrespect him. To the other empire agents, Vader can sometimes come across as a figure of ridicule because of his faith in an old religion, as he's one of the last Force-users in the galaxy. To his subordinates, he's a fearsome and enigmatic man with strange and unnatural abilities, something of an evil sorcerer. He is also quite treacherous, at one point proposing to Luke that they could overthrow his own mentor the Emperor and rule the galaxy together as father and son. However, at the end of his life he shows some guilt and remorse.
Vader's control over the Force allows him strong telekinetic abilities, including the ability to choke an enemy to death using only his mind. He also has some precognitive abilities, enhanced senses and reflexes, and is an extremely skilled military strategist, lightsaber duelist and pilot. As a cyborg, he also possesses superhuman strength and endurance.