The short answer
Claim preclusion (res judicata) bars relitigating a claim that was, or could have been, decided in an earlier case between the same parties. Issue preclusion (collateral estoppel) bars relitigating a specific issue that was actually decided before.
Where it fits in your case
It is raised as a defense, often in a section 2-619 motion, when a new suit follows an earlier one.
What to expect
Claim preclusion requires a final judgment on the merits, the same parties, and the same cause of action.
A transactional test. Illinois treats claims arising from the same facts as the same cause of action, even under different legal theories.
Issue preclusion can sometimes apply against a party even in a case with a different opponent.
What we need from you
Pleadings, orders, and judgments from any earlier case or arbitration involving the same dispute.
Common misconceptions
“A new legal theory is a new case.” Not if it arises from the same transaction.
“Only lawsuits count.” Arbitration awards can also be preclusive.