Example: Joe gives land to Mike, but if Harry graduates from law school, to Harry. Harry has an executory interest which he can realize by graduating from law school; Mike owns the land subject to Harry's executory interest.
Example: Joe gives land to Mike, but if Harry graduates from law school, to Harry. Harry has an shifting executory interest because his graduation from law school divests Mike of ownership.
Example: Joe gives land to Harry, if Harry graduates from law school. Harry has an springing executory interest because his graduation from law school directly divests Joe of ownership.