Villains are the antagonists of their respective stories. There motives are a unique as their personalities. Some seek vengeance, others money or even more devious, utterly dark motives. Whatever is the case they are essential to the story as without a villain there is no plot.
For a more in-depth description of what makes a character a villain please refer to the following villain's wiki page which describes in detail the evolution of villainy as well as what does it takes for being called a villain.