The villain usually is the antagonist (though can be the protagonist), the character who tends to have a negative effect on other characters. A female villain is occasionally called a villainess (often to differentiate her from a male villain). 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.
Villain comes from the Anglo-French and Old French vilain, which itself descends from the Late Latin word villanus, meaning "farmhand".The same etymology produced villein. It referred to a person of less 
than knightly status and so came to mean a person who was not chivalrous.