It takes a supervillain to mess with a superhero.
Supervillains are the much more powerful, (usually) much more developed, much harder to defeat/kill, much more intelligent, much more insane versions of your everday villains. Your everday villains may be enough for an episode of NYPD Blue or perhaps a few pages of a comic book showing everyone's favourite superhero whooping major ass but you don't usually see in-depth drama spanning years on end when Batman goes up against a purse-snatcher.
Supervillains are those that give superheroes a run for their money, send your every day heroes bolting into hiding, calling for back-up, or into the great unknown. Supervillains KILL. Supervillains often have superpowers, though this isn't required (just as some superheroes don't have superpowers). Supervillains are usually both insane and intelligent. Is it time for an example? I think it is...
"Hi, I use to be John Doe, an ambitious scientist in radiation research. Now I'm... THE RADIATOR! No, I'm not a car part, now shut up! While some ordinary folk become superheroes through radiation accidents, I had a lot of... issues... to work out, especially in the dealing-with-other-people department. Unfortunately for the world, instead of dealing with this well I became misanthropic.
Since my accident, I've been using my access to advanced scientific equipment and powers to radiate all my foes into a cancerous death in an attempt to conquer the planet and, through fascism, impose upon the good, God-fearing denizens of the world the manner in which I think they should live. Should I not be able to do this, I will attempt to destroy the world, taking myself with it in a firely ball of misplaced vengeance. Ha ha ha ha!"
As you can no doubt see, The Radiator, while lacking in a quality name, is evil, superpowerful, superintelligent, and quite a danger to the world. It will take a superhero (occasionally several) quite an intense battle to stop The Radiator, after which he will somehow escape justice (if he is taken into custody, he will escape later thanks to his superior intelligence) to attempt world domination/destruction again at a later date at least once. As with some superheroes, even killing some supervillains won't stop them. Some idiot will find some way of bringing the deceased supervillain back to life with the intent of working with him or her, only to either killed as soon as said supervillain gets the chance or used and then killed (or abandoned to death but saved by the appropriate superhero who the idiot then helps out of gratitute, seeing the light, etc.).
Occasionally, there's a supervillain with a much less global plan of doom. Sometimes there's a supervillain merely interested in destroying a particular superhero or city. Sometimes there's a supervillain who has created his own arch nemesis. Sometimes there's a supervillain who creates more supervillains, who may even fight against the first supervillain. However such supervillains show up and behave, they're almost always "evil" and they're always much more well developed than Joe Blow, the thug who died five minutes into the film.
Some supervillains you may have heard of:
Agent Smith/The Matrix AI/msg
Attila the Hun
Bloody Mary/Typhoid Mary
Bob the Killer Goldfish
The Borg Collective
EDB (but we love it just the same)
Evil the Cat
the evil Queen Pulsating, Bloated, Festering, Sweaty, Pus-Filled, Malformed, Slug-For-A-Butt
Jabba the Hutt
Jack O' Lantern
Khan Noonian Singh
Kraven The Hunter
The Mother of Invention
Sin Eater II
me with more and I will add them.
Last updated at 06:54 EUT on 15 October, 2001.
Contributions to the list so far are from myself, Lord Omar, Master Villain, and Josef Crazy (who added so much he should be considered an important co-author of this fiesty write-up of doom).