The Antagonist is the opposite number to The Protagonist. This is because the antagonist exists for the purpose of opposing the efforts of the Protagonist. They don't have to be a Villain, or even morally objectionable in any sense (though they very often are); they merely have to oppose, block, or impede the Protagonist. For example, if the Protagonist is evil, then the Antagonist is the one standing between them and their goals. The Antagonist usually provides the conflict and thus the dramatic and emotional tension in the story. Because of this, the Antagonist is about as Omnipresent as the Protagonist, though there are stories that have No Antagonist.
There's the Sliding Scale of Antagonist Vileness for, well, how vile an antagonist can be, ranging from fairly sympathetic to outright evil.