How should black meet 2.d4 in the Sicilian Defense? When playing the Sicilian Defence, what should black do when white's second move is: 2.d4

Should black take the white pawn, or should he continue to develop, by say moving pawn to e6?
In classical chess openings, the first thing white tries to do is build a pawn center with pawns on both e4 and d4. So the first goal of 1.e4 is to follow up with 2.d4, and the first goal of 1.d4 is to follow up with 2.e4. Of course, there is an opponent, so usually white has to shift plans quite quickly, but that's the classical theory.

Black can react in several different ways. If he attacks pawn e4 right now, then white can't really move 2.d4; that's what 1...d5 and 1...Nf6 do. 1...e6 and 1...c6 prepare 2...d5, so that black also gets a foothold in the center. Moves like 1...g6 let white go right ahead and hope that black will be able to sabotage white's center later and use the empty space left behind. 1...e5 puts black's own pawn in the center, makes 2.d4 exd4 possible and opens up lines for the bishop and queen: the classical best move.

1...c5 does slightly less than 1...e5: if white does play d4, black can chop off the pawn, and it opens up a diagonal for the queen, but no minor pieces are freed. The point is that that e-pawn is a center pawn, and therefore more valuable than the c-pawn: if black can trade c for d, he'll have one more pawn in the center. It's a very long term idea and he'll suffer first because he is behind in development, but the extra center pawn is helpful in defence and if he survives he'll probably have a better pawn structure.

So the whole idea of the opening is to play 2...cxd4 after 1.e4 c5 2.d4. 2...e6 would be followed by 3.d5, when black can't trade c for d anymore and white gets a slight space advantage.

This also explains the three main ways to play white against the Sicilian: fight it head on by still going for d4 (either 2.d4 or 2.Nf3 3.d4) and try to destroy black using the development advantage, prepare d4 with 2.c3 ("the only move that takes the Sicilian seriously", van der Sterren), or all the other plans that use d3 instead of d4, trying to show that white can do without d4 and that therefore 1...c5 is silly.