The Benoni defense is a closed chess opening and belongs to the family of indian openings and is sometimes called Benoni-Indian Defense.
The Modern Benoni Defense starts usually with the moves:1.d4 Nf6 2.c4 c5 3.d5
White plays d5 to gain space in the center.
If Black plays c5 right away like 1.d4 c5 2.d5 then this is the Old-Ben-Oni Defense, which has the same pawn structure, just the knight move is delayed.
White does not capture the pawn on c5 because Black would regain it later by playing 2...e6 or 2...Qa5+.
If Black plays the modern Benoni 1.d4 Nf6 2.c4 c5 3.d5 and then 3...b5 now, which is a pawn sacrifice, we are entering the realms of the Benko Gambit, where Black seizes the initiative on the queenside. This opening is recommended if you are an attacking player.
If Black continues quietly with 3...d6, 3...e6 or 3...g6 then this leads to the Benoni main lines.
As the position in the Modern Benoni is asymmetrical, it is the right setup for chess players who play an agressive game and play for a win. Players who like a quiet positional game should avoid this opening. This can be done by playing 3.Nf3 which leads to a quieter positional game.
Tip: If you play White, then play the Taimanov variation, which is pretty hard to handle for Black. Taimanov Variation: 1.d4 Nf6 2.c4 c5 3.d5 e6 4.Nc3 exd5 5.cxd5 d6 6.e4 g6 7.f4 Bg7 8 Bb5+ Nbd7? (correct move is 8...Nfd7) 9.e5!
If you have Black and want to play the Benoni Defense, then avoid the Taimanov Variation by varying the move sequence. Play 1.d4 Nf6 2.c4 e6 (not 2...c5) 3.Nf3 c5 because white has a knight on f3 now and cannot move the f-pawn. This makes the center break 9.e5 later on impossible. If White plays 3.Nc3 (not 3.Nf3) instead you play 3...Bb4, which is the Nimzo-Indian Defense.