Going from a standing up position to sitting down. Keep your back straight or arched and don't let your knees go past your toes. Go down until your thighs are about parallel to the floor and go up. Here's a link with visual aids.
http://www.exrx.net/WeightExercises/Quadriceps/BBSquat.html
http://www.exrx.net/WeightExercises/Quadriceps/BBSquat.html