There are three game types available that effect the flow of a game, and these are: play against the computer, play against a local human, and play a human by email. Each of these different game types has it's own unique flow of control that require the participents in a game to do things in a certain sequence.
Here we discuss how to play a game with the computer as an opponent.
Here we discuss how to play against another human while sharing the one computer.
Here we discuss how to play against another human by emailing the saved game file back and forth.