Villains and Their (Nefarious) Purposes.

Note: Summary at the bottom.

Villains are crucial to any good work; attempt to find a good piece of fiction you like with no Villain at all. In real life, of course, vilifying people (or just things) is common. A Villain can be defined by three things, usually.

From the average Villain's point of view, it is all about the Me. Most people think about the Us (Themselves, and those whom they know and care for) while true Heroes think about Them (The people around them, others - at the expense of themselves, and even for those they don't personally know already.) But for your average Villain, it's the Me - My needs, My wants, My desires, at the expense of My enemies - and anyone else, if need be. This is what gives Villains their callous repulsiveness - and this is ALSO what makes Villains who work towards other goals so interesting. The Me drives people towards the first trait of most of Villainy: Evil.

On the other hand, not all Villains are evil. However, another facet is this: Villains are active. Heroes, by and large, tend to react to evil, restoring the status quo, or at least preventing an 'unnatural' shift. This means first that the Villain will usually get halfway through their plan before being stopped, since the Hero starts with a handicap, but it also means that Heroes avoid moral questions, usually. The Hero is the driving force that ends the plot - but the Villain is the driving force that begins it, another case of opposing forces. This is the second trait of most Villainy: Activity.

But there is a third type of Villain - this kind need not be a bad person, or even aware of their role as Antagonist. Indeed, it need not be a person at all - 'it' can be an object or event just as easily, as it is in many war stories - the opposing force is just as often simple chance and danger as it is the opposing army directly, and certainly no single person in that army. This Villain is defined by mere opposition: the Antagonist opposes the Protagonist - morality, ethics, logic, none of these need enter. This is the most powerful trait of Villainy, and the most simple: Opposition.

Of course, Villains don't need to be Evil, nor do they need to be Active. They need only Oppose the Hero to be a Villain from the point of view of the plot; in a plot rallied behind an evil man, a good one is the 'Villain' even if few people would label him that. But this Opposition hides a more central truth - that even inactive Villains drive the plot of a work. Heroes are defined by their opposition even if what they oppose is, say, a mountain they must climb. However much it seems that a mountain could not be the Villain, it is: To the mountain-climber Protagonist, the mountain Antagonist defines him - if there were no mountain, then what? Our Protagonist would perhaps sit at home, wishing that his world were a bit less flat, because Villains represent challenge, and without challenge, life can get very boring indeed.


To summarize my wall of text:
Evil, Activity, and/or Opposition define villains. They need not have all three, however, to be an Antagonist.
Their role in any plot is to drive it by providing a challenge to be overcome, whether it is for Batman to foil the Joker's bomb-pie plot, or for a mountaineer to scale an unmapped mountain.
Villains are the neccesary opposing force: the positive charge to a Hero's negative. The Yin for Yang. The Chaos to Law (or vice versa.) They define our Hero, our Protagonist, as he opposes them.

Comments: Please do NOT use examples of real life people for Villains. Please do not bring up controversial works unless you are ready to be opposed.
Relevant Trope: http://tvtropes.org/pmwiki/pmwiki.php/Main/Villains
WARNING! TvTropes can be an addictive experience!
WARNING! The trope listed was not used in the creation of this post - if you read it expecting a continuation, you may be sorely disappointed.