The Greek Gift is a sacrifice of either a bishop or knight, most of the the time, a bishop. This sacrifice is made when your opponent castles, and does not have a knight on f6 protecting the weak h7 pawn. Works for either side.
Now, try this puzzle on your own!
For this position, it is white to play and checkmate in 7 moves!
Hint: The first move is a bishop sacrifice, leading with Ng5+.