The basic position of the Slav can arise after 1 Nf3 d5 2 d4 Nf6 3 c4 c6 or 1 Nf3 d5 2 d4 c6 3 c4 Nf6.
The most critical move for White is 4 Nc3. This makes it tricky for Black to develop his bishop to f5 or g4, as he would like to.
Now 4 …Bf5 is met by 5 cxd5 cxd5 (5 …Nxd5 6 Nd2 followed by e4 leads to White dominating the centre) 6 Qb3, which forces the clearly unsatisfactory 6 …Bc8, as 6 …b6 7 e4 dxe4 8 Ne5 is terrible for Black, as is 6 …Qb6 7 Nxd5.
Alternatively, 4 …Bg4 can just be met with 5 Ne5.
If Black wants to develop his bishop, his traditional approach has been to play 4 …dxc4. White is then advised to play 5 a4, which ensures that he can regain the pawn. Black now generally chooses between 5 …Bf5 and 5 …Bg4. White has several systems against 5 …Bf5, the most common being the sharp 6 Ne5, intending f3 and e4, and the solid 6 e3. The best approach against 5 …Bg4 is to play 6 Ne5 Bh5 7 f3.
A more modern approach is 4 …a6, the Chebanenko Slav, which intends to meet Qb3 with …b5 or …Ra7, therefore making …Bf5 or …Bg4 possible. White’s main options against this are 5 e3, 5 c5 and 5 a4.
An odd hybrid of the Slav and Gruenfeld is introduced by the move 4 …g6. White can simply play 5 cxd5 cxd5 6 Bf4, with the more comfortable game.
The last move worth mentioning is 4 …e6, which transposes to the Semi-Slav; this will be covered in a separate post.
Khalifman offers a repertoire against these lines in detail in volume 4 of his “Kramnik” series of books. Theory has naturally moved on since then, but the book still represents a good starting point for study.
Two more recent repertoires against the Slav based on 4 Nc3 can be found in Cox’s “Starting out: 1 d4!” and Schandorff’s “Playing the Queen’s Gambit – A Grandmaster guide”.
If White is not comfortable with the lines after 4 Nc3, a more sedate approach is introduced by 4 e3. Avrukh provides excellent coverage of this in his book.