Darth Vader's famous deep voice was done by James Earl Jones, the same actor who did the voice of Mufasa in the Lion King. The Darth Vader that we actually saw on screen was David Prowse, an actor and bodybuilder. Interesting side note: since James Earl Jones was the one saying Darth Vader's lines, David Prowse didn't even realize that Darth Vader was Luke Skywalker's father until he saw the movie!
Many of Darth Vader's stunts, like the lightsaber duels, were done by Bob Anderson, a professional fencer. And then, when Luke takes off Vader's helmet in Return of the Jedi there's ANOTHER actor playing Darth Vader, Sebastian Shaw.
But we're not done yet. When George Lucas re-released the original Star Wars trilogy he had to shoot extra scenes with Darth Vader, this time with actor and visual effects artist C. Andrew Nelson. Then, when Lucas released a prequel trilogy, he hired two more actors, Jake Lloyd and Hayden Christiansen, to play young Anakin Skywalker, Darth Vader's alter ego. That's SEVEN DIFFERENT DARTH VADERS.
Here's what Darth Vader sounds like without the voice-over.
(Sources: 1, 2)