Straight knees can be thrown with either the left or right leg. The base foot should arch, lifting the base heel off the ground, straighten the base leg to bring the large muscles of the leg into play, also shift hips forward while lifting knee upward and forward for maximum affect. The knee should have good forward penetration. Common targets include the thighs and abdomen of the opponent. Uncommon targets can include over extended limbs and even the head, if it presents itself.
Skip knees are basically straight knees thrown in alternating combination. The skipping motion allows for a full cocking of the rear leg before sending it in to knee. They are primarily used in the clinch after tying up the arms and securing the head. They can be used in combination with hook knees to destabilize the base of the opponent.
Hook knees are used to strike in a circular arc at the opponents legs and thighs. They are especially useful in the clinch. Hip torque is very important in generating force.