White takes the pawn. Black almost has to recover the pawn because of the control d5 exerts over his moves, but neither variation is very appealing:

• 3...Nxd5 allows 4. e4 when White has gained a tempo, the centre, and opened up lines for his pieces.
• 3...Qxd5 prevents white from obtaining a full pawn centre but results in 4. Nc3, winning a tempo anyway.

Because white is given a comfortable position regardless of what path black chooses, this opening is not seen very often at the professional level.

Black has also the option to play a gambit and aim at the center, with for instance 3...c6 4.dxc6 Nxc6 5.Nf3 e5!

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1. d4 Nf6 2. c4 d5

Wikipedia has related information at Marshall Defence.