The Alekhine Defense is a hypermodern defense against the ever so popular e4 opening from white.
Black looks to allow white to chase his knight all over the board with tempo gaining pawn moves that will control the center of the board. In exchange for the center control, black will look to undermine the overextended pawns from white.
The one thing black must always remember is that after his knight has been chased around he no longer can play passively and instead needs to attack the center that white has built up or he will be crushed by the pressure white can build up.
White has three main lines that he can choose from but they all start out with 1. e4 Nf6 2. e5 Nd5 3. d4 d6. From here things start to change and each variation takes on its on characteristics. White can choose from the very aggressive line of the four pawn attack where white will look to put his four central pawns in the center. He also might look to play the exchange variation that follows the four pawn attack but instead of the last pawn, instead opts to exchange with the d6 pawn. Black here can choose to play a sharp line capturing with his king pawn or play super aggressive and capture with his c pawn and really open things up.