Players choose which team they want to play on, without worrying about an unfair balance. If you have four players, you can have one versus three, two versus two, or have all four play against the computer player. Players choose from a variety of towers and creeps to use that have strengths and weaknesses.
Are the Soviets destroying you with their laser turrets? Toss the bulkiest creep you can. There's an opening between towers that leads right to the launch pad? That's a job for the fastest option on the team.