Darth Vader's costume is one of the most iconic designs associated with both Sci-fi and modern cinema. Vader's design is 70's early steam-punk tech, but it has become so iconic it has transcended that period. The armor was essentially a mobile life support system that Vader was forced to wear to compensate for the serious injuries he suffered after his duel with Obi-Wan Kenobi. Both the original and new costume were designed by Ralph McQuarrie, who was instructed by George Lucas to sketch "a tall and imposing figure, wearing an exotic-looking black armor". Originally, Darth Vader's costume was to have no helmet as Lucas envisioned that, instead of proper headgear, Vader's face would be covered by "a black silk scarf."
The eventual costume that was created for Vader was mocked up from an original World War II Nazi German Stahlhelm helmet bearing a gas mask, a monk's cloak, a leather undersuit, and a metal breastplate. TheSwedishBed [via Joblo] recently ran an excellent article on re-imagining a different look for the Sith Lords costume wherby artists and illustrators created their own versions using only the very first passage found in the original Star Wars script.
INTERIOR: REBEL BLOCKADE RUNNER — MAIN HALLWAY.
The awesome, seven-foot-tall Dark Lord of the Sith makes his way into the blinding light of the main passageway. This is Darth Vader, right hand of the Emperor. His face is obscured by his flowing black robes and grotesque breath mask, which stands out next to the fascist white armored suits of the Imperial stormtroopers. Everyone instinctively backs away from the imposing warrior and a deathly quiet sweeps through the Rebel troops. Several of the Rebel troops break and run in a frenzied panic.