Volga Gambit1.d4 Nf6 2.c4 c5 3.d5 b5
With his last move Black is offering a pawn sacrifice in order to open lines on the queenside and to become active there permanently. White can accept the gambit or decline it. The Volga Gambit is a serious opening which offers Black chances of counterplay.
Volga Gambit Accepted: 1.d4 Nf6 2.c4 c5 3.d5 b5 4.cxb5 a6 5.bxa6
In the Volga Gambit Accepted the white c-pawn gobbles everything up as far as a6. After that Black can immediately take the pawn with his bishop or first fianchetto his dark-squared bishop to g7 and capture the pawn on a6 later on. In all cases he tries to exploit the open lines on the queenside. In the meantime White develops his minor pieces and often occupies the centre with e2-e4.
Volga Gambit Declined: 1.d4 Nf6 2.c4 c5 3.d5 b5 4.Qc2
White can very well decline the Volga Gambit by developing his queen to c2. He is planning e2-e4 with a total occupation of the centre and if Black exchanges on c4 then he recaptures the pawn later with a minor piece instead of moving again with the queen.