The Queen's Gambit Declined is a chess opening which starts with 1.d4 d5 2.c4 e6. It is an old opening, and one of the most important. It is often used in top-class play. In this opening, called QGD for short, the struggle for the centre is the strategy of both players. Unlike the Queen's Gambit Accepted, Black keeps his pawn on d5 protected. A simple example:
position after 10...Bh4
8.Bd3 Nbd7 (Black could play 8...Ne4 here)
Here White has more space, and Black has to decide how to develop his pieces. It is easier to play the white pieces in this kind of position. However, overall, black players do as well with this defence as with most others. Best for Black in this position is 10...Ne4 or 10...Re8.