Chess Opening Theory/1. e4/1...c5/2. Nf3/2...Nc6

Black develops the knight to its natural square c6, where it controls the centre. This move is sometimes called the old Sicilian. Black forestalls a decision on a pawn move for a little longer: if they get to play ...d5 in one move later, they will have saved a tempo by not playing 2...d6.

3. d4 continues with the open Sicilian plan to open the position. After 3...cxd4 4. Nxd4, the most common lines include 4...g6, the accelerated dragon, 4...e5, the Kalashnikov, or 4...Nf6 5. Nc3 where 5...e5 is the Sveshnikov and 5...d6 is known as the classical Sicilian.

3. Bb5 is the Nyezhmetdinov-Rossolimo attack, and is the major side line to the open Sicilian.

3. Nc3 is a waiting move. White usually intends their knight for c3 eventually: by playing it now White waits to open the centre with d4 until they have more information about how Black intends to play. However, Black can capitalise on the hesitation with 3...e5, and White won't be able to get in d4 after all.

3. c3 is a delayed Alapin variation (1. e4 c5 2. c3), preparing to build a big centre.

3. d3 intends to play in a closed manner after all, with g3 and Bg2.

3. Bc4?!, a delayed Bowdler attack (1. e4 c5 2. Bc4?!), is a common club level sideline. This is not a critical line as Black can alleviate the bishop's pressure on f7 with ...e6.

1. e4 c5 2. Nf3 Nc6

Wikipedia has related information at Sicilian Defence.