A villain is an "evil" character in a story, whether a historical narrative or, especially, a work of fiction. The villain usually is the bad guy, the character who fights against the hero. A female villain is sometimes called a villainess. Random House Unabridged Dictionary defines villain as "a cruelly malicious person who is involved in or devoted to wickedness or crime; scoundrel; or a character in a play, novel, or the like, who constitutes an important evil agency in the plot."
What makes a villain a villain is down to personal opinion on what "good" and "evil" is but there are a few acts that are universally accepted as villainous: betrayal, murder and deception as well as abuse (especially sexual abuse) - also it is an almost universal acceptance that harming a child is an "evil" act so any character who is deliberately cruel to a child more than once would be considered a villain. Also in most cultures it is considered extremely dishonorable (and often illegal) for a man to hit or other abuse a woman, of course this goes both ways but it is seen as especially immoral when committed by a male.
Also, many villains do not see themselves as "evil" - only appeaing as such to the 'good' guys - and may have just goals but go about their deeds in immoral, even wicked, ways - in fact the cackling maniac intent on destruction for the sake of destruction is a character-type that while still popular in fiction is beginning to fade away in favor of villains that display the same varied depths of emotions as heroes do: making them more three-dimensional characters than they were originally.
Evolution Of Villainy
In the early days of writing and media it was common for a villain to be a cackling madman with little to no redeeming qualities however as audiences matured as society in general became a less morally absolute regime the idea of villainy was changed drastically - it became popular for a villain to share similiar qualities to a hero, this gave birth to the anti-hero and anti-villain as well as the tragic villain. In fact in modern times many writers are encouraged to steer away from outright villainious individuals and instead are asked to create antagonists that are more three-dimensional so as to serve a wider audience - it is worth noting a lot of villains in cartoons and children's media however retain the stereotype of being totally wicked, this is due to children's shows often having a more moral absolutist way of thinking (such as good and evil etc).