You should read Starting Strength, by Rippetoe and Kilgore. It goes into a lot of detail on correct form in performing barbell squats, and it also goes into det

You should read Starting Strength, by Rippetoe and Kilgore. It goes into a lot of detail on correct form in performing barbell squats, and it also goes into detail about what causes knee injuries. Short version: a full deep squat, with the top of the thigh parallel to the floor, is fine. Partial and incorrect squats are bad.