Alekhine's Defense is a chess opening that is named after the Russian-born French chess player Alexander Alekhine. It is a versatile and unbalanced opening that is favored by players who like to play aggressively and take control of the center of the board.

The Alekhine's Defense is a response to White's move 1. e4. In this opening, Black immediately challenges White's control of the center by playing the move 1...Nf6. This move attacks White's pawn on e4 and forces White to either defend it or, the much more common and main line of pushing with 2.e5.

The main line follows after: 1. e4 Nf6 2. e5 Nd5 3. c4 Nb6 4. d4 d6. This is the position presented by the board on this page. From here, white has a few options. The first, very aggressive option is 5. f4. This expands further the pawn control for white, but comes with huge weaknesses on the diagonal towards the king.

The other option is to trade pawns. This is a concession of sorts, and gives black more space and stake in the center. Against both of these options, black gets a good game. You can explore both of these options, and their main lines, with the PGN below, or by watching the explanatory video provided.

In conclusion, Alekhine's Defense is a versatile and unbalanced opening that can be difficult to defend against. By controlling the center of the board and attacking White's pawn on e4, Black can take the initiative and albeit have less space, always maintain pressure on the center.