The villain. The main bad guy. Joker to Batman. Lex Luther to Superman. The Antagonist is every story. Or is he? The Merriam-Webster Dictionary defines an antagonist as “one that contends with or opposes another; Adversary; Opponent.” A villain is defined as “a deliberate scoundrel or criminal; a character in a story or play who opposes the hero; one blamed for a particular evil or difficulty.” So while every villain is an antagonist not every antagonist is a villain.
For instance, an angry mob after the protagonist because they believe he committed crimes actually perpetrated by the villain would be an antagonist. The mob isn’t sided with the villain but still provides an obstacle the protagonist faces.
The villain, on the other hand, is the main bad guy. We should see plenty of this character, or his machinations, throughout the book. This does not mean he should show up, laugh evilly, and swoop away before the protagonist can do anything. It can only be small instances that you hear about the villain or someone you thought normal in the beginning turns out to be the villain. Your story should have clues throughout to point to that person as the villain.
Villain or not, the antagonist must present a challenge the protagonist overcome. There needs to be tension and the stakes must be high. If the reader doesn’t feel there is a chance the protagonist will fail then they will most likely grow bored. I have heard this lead to books being tossed across the room.
So in the interest of prevent novel abuse, make sure that you protagonist is hung from a tall tree and have plenty of rocks to throw at him.