Final answer:
An ex post facto law is a law that can punish an act that was not a crime at the time it was committed or increase the severity of punishment after the act. A bill of attainder is a law that pronounces someone guilty without a trial and imposes punishment. Both are prohibited in the United States.
Step-by-step explanation:
An ex post facto law is a law that has a retroactive effect, meaning it can be used to punish an act that was not a crime at the time it was committed or to increase the severity of punishment after the act. On the other hand, a bill of attainder is a law that pronounces someone guilty of a crime and imposes punishment without a trial. These laws were used by the British Parliament during the Revolutionary period and are prohibited in the United States.