What is the difference between an affirmative defense and a denial?

Answer 1

A denial does just that it denies the Plaintiff's allegations and the burden of proof is still on the Plaintiff to prove the prima facie case.

An affirmative defense does not deny the allegations but asserts a defense that would negate the legal effect of the Plaintiff's cause of action. The burden of proof in an affirmative defense is on the Defendant.

An example would be a breach of contract case. The Plaintiff claims that he had a contract with the Defendant, and Defendant did not perform the contract. A denial would say "We never had a contract" and the Plaintiff would have to prove the existence of a contract. An affirmative defense would say "Yes, we had a contract, but that was 20 years ago thus the action is barred by the 10 year statute of limitations." Then the burden of proof is on the Defendant to show that the contract falls outside of the statute of limitations period.