Queen's Gambit

The Queen's Gambit is a chess opening which starts with 1.d4 d5 2.c4. It is not really a gambit, because the pawn taken cannot be held. For example (simplest line):
1.d4 d5
2.c4 dxc4
3.e3 b5
4.a4 c6
5.axb5 cxb5
6.Qf3 and wins.

 2...dxc4 is the Queen's Gambit Accepted. 5085 games in database.Chessgames.com
 2...e6 is the Queen's Gambit Declined. 14,635 games.
 2...c6 is the Slav defence. 17,801 games.
 2...Nc6 is Chigorin's defence. 836 games.

 Queen's Gambit Accepted 

Since the pawn can't be kept, Black (by taking the c-pawn) has given up the centre. The standard strategy then is for Black to mount a counter-attack on the White centre.
1.d4 d5
2.c4 dxc4
3.Nf3 Nf6
4.e3 e6
5.Bxc4 c5
6.0-0 a6
7.Qe2 b5
8.Bb3 Bb7
9.Rd1 Nbd7
10.Nc3 Qc7/Qb8/Qb6 (alternates)

Now the white centre is balanced by the black counter-attack.

 References 

Category:Chess openings