Sicilian: What Happens After The Immediate 5... e5?!
The Sicilian Defense: What Happens after the immediate 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5?!
This is a quite dubious move, trying to steal tempo from white. However, white can "takeback" the tempo by 6. Bb5+! Bd7 7. Bxd7+ Qxd7 when tempo is equal; black wasted time dealing with the check, and white wasted time dealing with his knight. Furthermore, that's why the Najdorf is effective; instead of the immediate 5... e5?! occurs: 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 making 6... e5 safe because white loses a tempo because the pawn on a6 keeps the bishop out of b5, though it leaves a big support point for a knight on b6.
The typical Najdorf:
The Schveningen, which is 5... e6 is also okay, though the Najdorf (shown above) variation is also alright.