In 1997, Episodes IV, V, and VI were re-mastered and re-released as 'Special Editions', with George Lucas adding special effects which weren't originally possible way back then in 1977 when the first Star Wars film came out. There were more scenes on Tatooine, including new ones of Sandtroopers and their Dewback ride which didn't happen in the original because they couldn't get the Dewback to work because of the sand in the desert.
In 2004, Lucas released the Star Wars DVDs and tweaked the films further. A notable and quite controversial change was to a scene at the end of Episode VI, when the spirits of Obi-Wan Kenobi, Yoda and Anakin Skywalker look upon the Rebels' celebration. The original actor who played Anakin in this scene (Sebastian Shaw) was replaced by stock footage of Hayden Christensen. IMHO, this was wrong because old Yoda and old Ben were in spirit form because that's when they became one with the Force. Anakin did not become one with the Force until he as Vader dispatched Emperor Palpatine and saved Luke, so how can it be a young Anakin standing next to his elders. This scene really irks me!
This Sebastian Shaw as Anakin Skywalker version was released as part of the Masterpiece Edition sets that came out in 1998. There were also C3PO with detachable limbs (from Episode V when he was blasted into parts on Cloud City) and Aurra Sing versions released.