What is the idea of 3. d4 in the Sicilian?

Active piece play and attacking possibilities.  Best approach to combating the Sicilian(haha c3'ers!!).  

I won with c3 until I started playing players who knew what to do; then I stopped winning with c3. 

Black will have a better pawn structure and White will have a space advantage and more active pieces.

In the open Sicilian (d4 exd4)

Black has two center pawns (to white's one), a half open c file, and a solid position.  Black likes to seek play on the queenside with b5 and a rook on c8 and or play d5 to open the position.

White has space and easy / faster development.  White likes to cramp black (prevent the b5 queenside expansion and the d5 break) and or get an attack on black's king.

Each side has their own ideas so often both players are willing to enter into it.  If white doesn't like it he'll play an anti silician like 2.c3 which can turn into a french or a closed Sicilian characterized by Nc3.  White may also try a King's Indian Attack setup i.e. e4, d3, g3, Bg2, 0-0.

The primary purpose of C5 is to prevent D4, hence by playing D4, you have defeated black's main theme; make no mistake after CxD4 and NXP the position is very sharp and requires accurate play  to convert the point.

3.d4?? (Where white trades a powerful center pawn for a less important black pawn: a positional blunder)

The idea of 3.d4 is to open the game to take advantage of a development lead as 1...c5 is not a development move (doesn't help any of the Black pieces enter the play except the Queen)

The idea of 3.d4 is to prevent 3. ... e5. For example 3. ... e5, then 4. dxe5 dxe5 5. Nxe5. Black loses a pawn and has no compensation. White is better.

black:

no broad pawn center so dont have to worry of e4 d4 pawn center formation, or center pawn advancing clamping down position or attacking knights, or making prelude to create advanced pawn or strong attack.

white:

after 3... cxd4, white has lost control of c5(unlikely ever want to play b4) and some e5(sometimes there is f4). He has loose some of dark square control in the center, various black defenses attempt to profit from this d4 pawn missing in the form of ...Bc5, or ...Qb6 even ...Qc7(this one having eye e5 square). Without pawn barrier king side fiancheto seems good hence dragon variation. Or black can try ...e7-e5 to try to directly control d4 with a pawn, for example Sveshnikov, and some lines of the narjdorf.

White if fully aware of fragility of center thats why in the dragon he protects the knight in d4 as much as possible with Be3, Qd2 and 0-0-0.

Because dark square are somewhat weak after ...cxd4 the dark square bishop is good defender of that, although in many variations it can be traded you shouldnt do it without thinking.

d4 pawn is gone but e4 pawn is proud. Unless black tries ...e7-e5, wish its not without its drawbacks, can make ...d7-d6 or ...e7-e6 or perhaps both. not much near to the frontier line concept of nimzovitch or third rank, giving white a significant space advantage. Even the absence of d4 pawn has bright side  as white now have semi-open file for his rooks in the center, besides having the knight nicely ensconced on d4.

Because white cant just sit behind strong center formation and try to gradually suffocate its oponent that happends in some e4 d4 formations, white empower e4 pawn with f2-f4, wish in some cases follow with e4-e5 as a pawn sacrifice to disrupt blacks game, or after black castles kingside the pawn on f4 supports a pawn storm with g2-g4-g5. Assuming black is kept pinned down , this pawn advance can be effectively conbined with kingside castling by white. Another common form of pawn attack employed by white is f2-f3 and g2-g4, combined with queenside castling.

Nonetheless, black trust in the solidity of his position when he choses the sicilian, despite the fact that white pieces will normally have greaters scope for the activity at the start of the middlegame.

3 winning scenarios for black: 1) bad sacrifice, 2) white understimate danger, 3) initiative gone leaving pawn better placed for endgame. So as white it demands fighting spirit.

taken not so literally from GM Neil McDonalds book, "Starting out e4, a reliable repertoire for the improving player".

Many people have already addressed this issue, though I think it will help even more if we showed you instead of throwing out a combination of letters and numbers... so I shall insert a board with light commentary on the first few moves of the openings Scotch Attack vs Open Sicilian Defense.

The Scotch Attack is one of the very typical e4 openings that follows the same opening sequence you are inquiring about.

Hope this helped. 

Kiwi did a very nice job of answering part of you question. The part that he left unanswered is the theoretical reason why 3.d4 is played. There 2 theories of chess. The Classical Theory and the Hypermodern Theory.

Classical Theory: Control the center by occupying the center (d4,e4, d5, e5) with your pawns and pieces.

Hypermodern Theory: Control the center with the power of your pawns and pieces. With this method you do not make your pawns and pieces into targets in the center for your opponent to attack. You will notice that Black is applying the Hypermodern Theory with every move in this opening.

Having written all of the above, it becomes clear that the idea 3.d4 in the Sicilian is the application of the Classical Theory control the center by occupying the center with your pawns and pieces. The move 3.d4 is a direct occupation of the center by White with his pawn.

There were two theories of chess in the 1930s. Chess has moved on a bit since then.

And black is definitely not the one playing "hypermodern" chess -- his most important trump is his two center pawns, usually controlling centrale squares from d6 and e6.

kikvors wrote:

There were two theories of chess in the 1930s. Chess has moved on a bit since then.

And black is definitely not the one playing "hypermodern" chess -- his most important trump is his two center pawns, usually controlling centrale squares from d6 and e6.

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Yes, you are right. In practical play over the board today as well as in the past it is a combination of the 2 theories that is applied. Yes, after 3...cxd4 4. Nxd4, Black enjoys a 2 vs. 1 central pawn majority. But, you will notice that Black's pawns are at d6 and e7, and if Black chooses to play for example the Najdorf/Scheveningen style he will play ...e6. You will notice that this move is still in keeping with the Hypermodern Theory. By contrast after 4.Nxd4 you will notice that White's N is at d4 and e pawn is at e4. The piece and the pawn are occupying central squares in keeping with the Classical Theory.

The 2 vs. 1 central pawn majority that Black has usually becomes an important factor in the endgame. But, it does influence middle game play due to thematic d5 square in the Sicilian opening. The move ...d5 by Black, occupying the center with his paw, at the appropriate time, when Black is assured of controlling the center is an application of the Classical Theory.

That whole line you showed is junk.