Substantive: terms are grossly unfair or favor one party disproportionately, e.g. Rumplestiltskin, "I get your first-born."

Procedural: terms are agreed to by unfair surprise, e.g. The Hitchhiker's Guide to the Galaxy: "It was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying 'Beware of the Leopard.'"