Tarrasch Defense
Queen's Gambit
1.d4 d5 2.c4 e6 3.Nc3 c5 4.cxd5 exd5 5.Nf3 Nc6 6.g3 Nf6 7.Bg2 Be7 8.O-O O-O
Black aims to create dynamic counterplay at the cost of an isolated pawn.
Fun fact: In his book Die moderne Schachpartie, Siegbert Tarrasch marked the usual Queen's Gambit Declined moves with question marks and gave his own 3...c5 an exclamation mark. Source
Practice this lineThe idea
Black strikes at the center at once with 3...c5. After 4.cxd5 exd5, Black has an isolated pawn on d5, but in return gets free development, open lines, and a queen's bishop that is no longer shut in.
In the main line, 5.Nf3 Nc6 6.g3 Nf6 7.Bg2 Be7 8.O-O O-O, White's bishop on g2 points straight at the d5 pawn, and the whole game revolves around it.
Plans for White
Pile up on d5. The fianchettoed bishop, the knights and often a rook on d1 all aim at it, and Black can't defend it with a pawn.
The modern main move is 9.Bg5, which threatens to take on f6 and remove one of d5's defenders. Trades favor White: the fewer pieces there are, the weaker the isolated pawn becomes.
Plans for Black
Use the activity the isolated pawn gives you. Black's pieces have open lines and good squares, especially e4 for a knight. Answer 9.Bg5 by resolving the center with 9...cxd4 first.
Avoid unnecessary trades and keep the pieces active. With accurate defense, Black can often hold even a slightly worse endgame.
Watch out for
For Black, the danger is the d5 pawn simply being lost. Defenders like the knight on f6 can be removed by Bg5xf6, and if Black passively waits, White's pressure keeps building until d5 falls. Many Tarrasch games are lost without an obvious mistake, through the structure alone.
For White, the danger is being too slow. If White doesn't put real pressure on d5, Black's active pieces and central pawn give Black the easier game.
History
Tarrasch called it the only correct answer to the Queen's Gambit, but after Aron Nimzowitsch and Akiba Rubinstein showed how to exploit the isolated pawn, it was rarely played for decades.
Boris Spassky revived it in the 1960s and used it in his 1969 world championship match against Tigran Petrosian. Garry Kasparov played it in his 1984 Candidates final against Vasily Smyslov, who couldn't break it, but Kasparov gave it up after losing two games with it to Anatoly Karpov later that year. More recently, Daniil Dubov has played it at the top level.