A more thorough explanation:

An ex post facto law is a law that retroactively applies to an action that was legal when it was committed, especially if it negatively affects a person's rights. For example, if a law is passed today that makes it illegal to drive a red car, and someone who drove a red car yesterday is punished for it, that would be an ex post facto law.

Ex post facto criminal laws are prohibited by the U.S. Constitution, but retrospective civil laws may be allowed. A retrospective law is a legislative act that looks backward or contemplates the past, affecting acts or facts that existed before the act came into effect. A retroactive law is not unconstitutional unless it (1) is in the nature of an ex post facto law or a bill of attainder, (2) impairs the obligation of contracts, (3) divests vested rights, or (4) is constitutionally forbidden.

For example, if a law is passed that changes the tax rate for the previous year, that would be a retrospective law, but it would not be an ex post facto law because it does not criminalize an action that was legal when it was committed.