The strap is the flexible, non-mechanical part of the belt, that goes around the wearer's waist. Can be made of leather, a textile, or a synthetic.
Within this is the loop, which is meant to keep the end of the strap in place, on the part that overlaps, once it's gone all the way around the wearer's waist.
The buckle collectively refers to the metal hardware that is affixed to one end of the belt. It can either be one piece, or can consist of multiple pieces such as in the image below.
The outer portion is called the frame. The bar that goes across is called the prong. When the other end of the belt goes around, it goes through the frame, as shown below.