The short answer
An affirmative defense is a reason the plaintiff should lose even if its allegations are true, such as a release or the statute of limitations. A counterclaim is the defendant’s own claim against the plaintiff.
Where it fits in your case
Both are generally filed with the answer. Adding a counterclaim later usually requires the court’s permission.
What to expect
Plead it or lose it. Affirmative defenses not raised in the answer may be waived (735 ILCS 5/2-613).
Independent claims. A counterclaim can continue even if the plaintiff drops its case.
Federal court requires counterclaims from the same transaction, or they are generally lost.
Insurance typically funds your defense, not your counterclaim.
What we need from you
Facts and documents supporting any claim you have against the plaintiff, and anything showing releases, payments, or relevant dates.
Common misconceptions
“We can always sue them later.” Related claims can be barred if not brought now.
“Listing every defense is safest.” Boilerplate defenses can be stricken.