Chess Opening Theory/1. d4/1...Nf6/2. Nc3/2...d5/3. Bf4/3...e6

This is Black's most popular response. This move reinforces the centre and opens up the dark-squared bishop. It does, however, pacify Black's light-squared bishop. The next couple of moves will likely happen on the queenside, since neither side has many attacking opportunities on the kingside.

The thematic 4. Nb5 is White's main response, threatening a fork on the c7-square. Black has two ways to defend against this: 4...Na6 is the most popular response. Black defends c7 with their knight and will soon kick out White's knight with c6 and reposition their a6-knight to c7 and subsequently e8, protecting the kingside. Alternatively, Black can opt for 4...Bb4+, which leads to a forcing line 5. c3 (5. Bd2? Bxd2 6. Qxd2 ⩱) Ba5 and the bishop is now protecting c7. If White continues chasing the bishop with 6. a4 a6 7. b4 Bxb4 8. cxb4 axb5 9. axb5 Rxa1 10. Qxa1 = the players have entered an endgame where White has a worse pawn structure, but more advanced pawns on the queenside.

Alternatively, White can play 4. e3, simply continuing development. This gives Black the opportunity of playing 4...Bb4, pinning the knight and possibly doubling White's queenside pawns, similar to the Winawer French. Alternatively, Black can continue 4...Bd6, preparing to trade dark-squared bishops.

1. d4 Nf6 2. Nc3 d5 3. Bf4 e6

Wikipedia has related information at London System.