The Dark Side exists in the Star Wars universe. It is based on the Force. The Force is what connects every living thing together. Basically, if one can use the force, they can manipulate objects (raising a rock), or even change peoples thoughts (convince them to give you a concert ticket for free). The Force is split into two sides, the Light Side and the Dark Side. The Dark Side is (as it sounds) the evil side. Although it promises quickly gained power, it also promises corruption, sometimes even leading to physical disfigurement. The Jedi most often use the Light Side to combat those who use the Dark Side.

Notable uses of the Dark Side:

Darth Vader (A.K.A. Anakin Skywalker): He is debatably the coolest villain ever. Maybe it's his cool calm manner, his overwhelming sense of authority, or just his awesome suit, but no one can honestly say that they weren't intimidated the first time they saw him.

The Emporer: Although he was technically Vader's superior, and was evil himself, he just never seemed as powerful as Vader, even though I would love to be able to shoot blue lightning from my fingers whenever I want.

Luke Skywalker: Yes, even the main hero of Star Wars turned to the Dark Side at one time. Fortunately, he was able to turn back to the Light Side, no small task for one of his power (the addiction grows for those strongest in the Force).