Fictional supervillains are the foils to comic-book superheroes and other fictional heroes. The typical supervillain is an evil genius. They are villains, but with extraordinary power or ability which makes them viable antagonists for the most gifted heroes.
It could be said that the first supervillain was Professor Moriarty.
Supervillains are often depicted as seeking to commit spectacular crimes and/or rule the world through terror from their secret underground lair. Some supervillains like Mr. Mxyzptlk prefer to simply torment the heroes for their own pleasure. Some supervillains like Magneto have laudable goals such as his desire to protect his people, mutants, from persecution, but his methods are so extreme and violent that superheroes have no choice but to oppose him.
Also at least until recently, most supervillains were not team players and prone to fight amoung their contemporaries. They're also typically highly vindicative with a great desire to kill their superhero enemies for interfering in their crimes, often to expense of more profitable ventures.
Examples of fictional supervillains include:
In fantasy the supervillains are typically Dark Lords.
This stereotype led to the real-life terrorist Osama bin Laden being portrayed in the media as if he was a supervillain. The Nazis are also depicted as supervillains in a range of fictional works, including the Indiana Jones stories.