Nb5 threatens Nd6+, where after trades the queen will be in a good position. This is why Black often plays 5...d6, where Black defends d6 with the bishop and the queen. The knight is, however, quite exposed, and is often kicked with a6, and as the b1 knight comes to c3 later on, Black threatens a fork of the knights with b5, and as when the c3 knight comes to the safe square d5 the e4 pawn is hanging, Bg5 is often played as well to pin the knight, and the bishop is sometimes traded.

1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 e5 5. Nb5

Wikipedia has related information at Sicilian Defence.