This is without weight. From an erect position you bend your knees until they are parallel to the floor and then back up again - this is one rep. Your hands can be placed behind your head or folded in front of you throughout the movement.
I recently started doing Hindu squats. This is a very fast, very intense exercise, where you move your hands in almost a "rowing" motion as you perform the squat. You also go lower than a standard squat, and do all the movements more on the ball of the foot.
So far, I can do 50 consecutive Hindu squats. Last week, I did two sets of 40, separated by about 5 minutes. My thighs hurt for two days after that.