Blokus is a simple, yet abstract game that doesn't have a random component to it. The game is always played with four players, with each player controlling a different color. Each player has 21 tetris-like game pieces that fit onto a grid-like board. The goal of the game is to get rid of as many pieces as you can.
Actual gameplay is best explained visually, but here goes:
Each player starts by placing one of their pieces on the board, beginning in their respective corner. When it is the first players turn again, they will place another blokus piece on the board, but it must only be touching the corners of the previous piece. The sides of the pieces cannot touch one another. Opposite colors can touch sides.
As players take turns placing their pieces on the board, it becomes increasingly more difficult to find spots for pieces to go. As soon as a player can no longer place a piece, they are out. As soon as no one can place any more pieces on the board, the player with the fewest pieces left after the game is the winner.
This may have been confusing, but the actual game makes a lot more sense if you can look at it while it is being explained.