The trap, named after James Mortimer, begins with Black playing the Berlin Defense to the Ruy Lopez. It is a true trap in the sense that Black deliberately plays an inferior move hoping to trick White into making a mistake. (Source: Wikipedia)
1. e4, e5
2. Nf3, Nc6
3. Bb5, Nf6
4. d3, Ne7
Black intends to reroute the Knight to g6. It is an inferior move, but it lays a trap.
5. Nxe5?, c6!
Black threatens 6....Qa5 winning at least 1 piece.
This is White's best reply, as it covers a5, and threatens 7. Nd6 mate.
6. .... d6!
7. Ba4, b5
Black forks the White Bishop and Knight, winning a piece for two pawns.