You can put in waypoints and stop points where all the teams stop until you give the go command. While this system is good in practice, in reality it is complicated and takes quite awhile to get used to. The first few mission allow you to use default plans, but after that you are on your own. It's this emphasis on up front planning that sets Rainbow Six apart. The wrong personnel and equipment will make it difficult or impossible to complete the assigned mission once you hit the ground.
Mission have a variety of objectives, but they are usually bomb disposal or hostage rescue.
Following the route you had already planned before the mission began, you go round eliminating targets as they appear. The missions are exceptionally realistic although the enemy AI is only average. Enemies will shoot on sight but they stick to their guard posts rather too much, with only a few that will occassionally follow you around a corner. One interesting twist is that if an enemy runs out of ammunition he will drop his gun and surrender. You can choose whether to blow them away or be a little nicer.