I assume you mean for black after 1.d4 d5 2.c4
A. dxc4 is the QG Accepted
A common theme in this one is white gets an isolated d pawn and extra space with attacking chances. Black's position is slightly restricted but certainly playable.
B.2. ... c6 is the Slav/Semi-Slav
3.Nf3 Nf6 4.Nc3 dxc4 is the Slav. Black gives up his presence in the center to develop his queen's bishop actively. Black has good piece control of the center, white occupies the center with pawns.
4. ... e6 is the Semi-Slav. 5.e3 Nbd7 6.Bd3 dxc4 7.Bxc4 b5 is the main line. Black launches the queenside pawns forward and creates active play. Black's position can become loose without careful play.
Many of the top players prefer this defense. It is a strong weapon in the right hands. Unfortunately, there is tons and tons of theory.
C. 2. ... e6 is the Classical QG Declined
C.1. 3. ... c5 The Tarrasch Defense This time, black takes an isolated pawn and pawn control of the center. The whole game becomes a battle over the d5 and c5 squares. There is a lot of theory to this one too.
C.2. 3. ... Nf6 4.Bg5 Nbd7 5.Nf3 c6 6.e3 Qa5 is the Cambridge Springs
Black tries to get active very early. I haven't had much experience with this one. The games that I have played I have either won very early from traps, or lost very theoretically from positions that lacked counterplay.
C.3. 3. ... Nf6 4.Bg5 Be7 5.e3 Nbd7 (4 and 5 are interchangable).
This is the Old Orthodox QG. It served Lasker and Capablanca quite well. 6.Nf3 0-0 White tries to avoid Bd3. 7.Rc1 c6 8.Bd3 dxc4 9.Bxc4 Nd5 This is Capablanca's maneuver. The idea is for the black side, with less space, to trade pieces. 10.Bxe7 Qxe7 11.0-0 Nxc3 12.Rxc3 e5 is one variation. The defense is solid but a bit dated. Not many players use it anymore. Just remember h6 does not really fit in in this system.
C.4. 3. ... Nf6 4.Bg5 Be7 5.e3 h6 6.Bh4 0-0 7.Nf3
C.4.1. Ne4 is the Lasker Defense Again black seeks exchanges to free his game. 8.Bxe7 Qxe7 9.cxd5 Nxc3 10.bxc3 exd5 is a typical line. The defense is very playable.
This site has a pretty good lecture on it
C.4.2. 7. ... b6 is the Tartakower It too is very solid. 8.cxd5 Nxd5 9.Bxe7 Qxe7 10.Nxd5 exd5 is one way the game can proceed.
Among the QGD with e6 choices, I'd say the Lasker or the Tartakower are the way to go.
There is one other problem with the black side of the QGD.
1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.cxd5 (The Exchange Variation) is a really solid white system.
After 4. ... exd5 5.Bg5 Be7 it can be really hard for black to get active play.
I hope this little overview helps.
You will obviously need more analysis.