An ex post facto law is one that is made retroactive to punish an act that wasn't illegal at the time it was committed. For example, if I didn't wear my seatbelt and wearing one wasn't a law, but it...
An example would be if a woman was driving in Illinois and the speed limit was 50mph. So she drove exactly 50mph. The passing police officer checked her speed and insured that she was going the good...
There is no such law. The term is a catch-all phrase for any law that changes the legal consequences of an action that occurred before the passage of the law.