Sorry I got the two films mixed-up. It was The Empire Strikes Back where Luke found-out about the connection between himself and Vader not Return Of The Jedi. Again sorry about that, I sometimes get them confused with one another.
Darth Vader is a very fearsome, dark and complex type of a character. When the audiences first met him in the film Star Wars, Vader seemed like just another tough bad guy. However, the sequel Return of the Jedi revealed an important and a very powerful connection between himself and Luke Skywalker and gave the audiences some information on Vader that later films in the franchise would use. Vader's character is a very highly interesting one because he goes from being a kick ass bad guy in the first film ever in the entire franchise to evolving to being a big hero in The Empire Strikes Back. In other words, Vader himself is a very powerful and dreadful force to be reckoned with and he's so awesome that he's wicked.