Simply put, the Courts fill in the blanks. Legislature passes a law and then as cases are decided and appealed the blanks are filled in. Easy example: Ohio legislature passes a law stating you can't drive a vehicle under the influence. Sounds simple enough right? Not so simple. What exactly is a vehicle? A car? Of course...what about a Segway? Hmm. That's where the court's come into play. Let's say Jimmy is arrested for a DUI on his Segway and a Jury finds him guilty. Well, Jimmy can appeal that conviction to the court of appeals. The court of appeals will then decide if a Segway is a vehicle. If they decide that it isn't, then going forward you can't be charged with a DUI on your Segway. The decision is only binding on the area which the that court of appeals controls. This is a simplified explanation so do some reading if you want to know more.