Two players take turns in droping coins into the board which is standing upright The goal is to be the first player to reach a row of four coins horizontally, vertically or diagonally. See this.
To drop a piece, click the entry field (on top of the board) in the according column.
Your game will be stored and identified by the URL. So after starting a game, you can always come back to it by going to the URL you got redirected to on game start.
The AI opponent
When playing against the computer, its strength is determined mainly by the time you allow it to think. You can set this to a value between 3 and 20 seconds when starting a game.
After calculating a move, the computer will remember it for future games, so by time it will have fast responses in more situations.