Classical Defense
Queen's Gambit
1.d4 d5 2.c4 dxc4 3.Nf3 Nf6 4.e3 e6 5.Bxc4 c5 6.O-O a6
Black develops calmly and gives the pawn back, striking at White's center with ...c5 while White regains it with Bxc4.
Fun fact: Wilhelm Steinitz used this idea in the 1886 world championship, giving the pawn back so that White would be left with an isolated pawn on d4. Source
Practice this lineThe idea
This is the main line of the Queen's Gambit Accepted: 3.Nf3 Nf6 4.e3 e6 5.Bxc4 c5 6.O-O a6. White has the pawn back and has castled. Black has developed, challenged d4 with ...c5, and prepared ...b5 to chase the bishop from c4.
If the pawns are exchanged on d4 and White recaptures with the e-pawn, White is left with an isolated pawn on d4. It gives White space and active pieces in the middlegame, but it can be a weakness in the endgame.
Plans for White
A common plan is Qe2 and Rd1, putting pressure on the d-file. When Black plays ...b5, retreat the bishop to b3 or d3.
Look for the central breaks. If Black is slow to castle, d5 or e4 can open the position. If you end up with an isolated d-pawn, keep the pieces on and play for an attack, since the pawn gets weaker as pieces come off.
Plans for Black
Continue with ...b5 and ...Bb7, putting the bishop on the long diagonal where the queen's bishop has none of the problems it has in the Queen's Gambit Declined. Bring the knight to d7 and castle.
Against an isolated d-pawn, blockade it, trade pieces, and attack it in the endgame.
Watch out for
Black's main risk is keeping the king in the center for too long. In the main line after 7.Qe2 b5 8.Bb3 Bb7 9.Rd1 Nbd7 10.Nc3, White meets Black's queen moves with 11.d5, opening the center while Black's king is still on e8. Finish developing and castle before starting queenside operations.
White's typical mistake is trading pieces when White has an isolated d-pawn. Every exchange takes away attacking chances and leaves the pawn easier to win in the endgame.