Two point is jumping or galloping position allowing the horse his mouth and back. The main idea is to bend forward at your hips and reach forward up the horse's neck, giving him rein. Galloping position is slightly different as you do bend as far forward, and keep a little more rein contact. It is sometimes referred to as "three-point" and is seen doing a hand-gallop, or controlled gallop. You want to keep your heels down, but NEVER stand in your stirrups.
Posting, also called rising trot, is going up and forward with your seat in the two beat rhythm of a horse's trot. A well-practiced posting trot can be done without stirrups (again, you shouldn't use your stirrups to rise) you should use your thighs. Hope this helps!!! :)
Galloping position/three-point: Redirect Notice
Jumping Position/half seat/two point: Redirect Notice