Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...c6/4. e4/4...dxe4/5. Nxe4/5...Bb4

This move is Black's best option to fight for an advantage. It forces White to either gambit the d4 pawn for activity, or to retreat the knight back to c3, "losing" a tempo in the process.

6. Bd2 is the main move, blocking the check and attacking Black's bishop at the same time. This does however block the white queen from defending the d4 pawn, allowing 6...Qxd4, which gambits the pawn in exchange for quick development and active pieces.

6. Nc3 is the Forgotten variation, a sideline where White holds onto the d4 pawn, keeping material equal.

6. Nd2? is a mistake, since it contains all the drawbacks of 6. Bd2 (losing d4) and none of the advantages (getting a very active dark-squared bishop). To add insult to injury, it also hampers the development of White's queenside by locking up the c1 bishop.

1. d4 d5 2. c4 e6 3. Nc3 c6 4. e4 dxe4 5. Nxe4 Bb4

Wikipedia has related information at Semi-Slav Defense.