Glossary
Counterclaim
A claim the defendant brings against the plaintiff, in the same case the plaintiff started. Counterclaims turn the defendant into a plaintiff (for the counterclaim) without having to file a separate lawsuit.
If the plaintiff sues you and you also have a claim against the plaintiff arising out of the same situation, you can raise it as a counterclaim in your answer instead of filing a separate lawsuit. In many courts, you have to: that's called a "compulsory counterclaim," and waiting and suing later is barred.
Counterclaims that arise from a different situation are "permissive counterclaims": you can include them in this case or sue separately, your choice.