The Nd5 sacrifice is a thematic idea in the Sicilian defense. Black accepting the sacrificed knight will open the center, accelerating White's attack. White often uses the sacrifice to open lines for his attack or to break through when he has a lead in development.
Sometimes when Black accepts the sacrificed piece, White will win by force, or at least win his piece back while maintaining his attack. Other times, White gets a powerful initiative for the sacrificed piece with an unclear position.
This sacrifice is especially common when Black spends a tempo to attack the c3-knight with ...b4 and when Black's king is uncastled.
Here are some examples: