What does Button mean?
Buttons are small, usually round fasteners sewn onto garments and used to attach two pieces of fabric together. There are three main styles of buttons: shank, sew-through, and stud.
Heddels explains Button
Shank buttons have a hollow loop on the back through which thread is sewn to attach the button to the fabric, while sew-through, or flat, buttons have either two or four holes through which the thread is sewn. The third type, stud or tack buttons, are the types found on jeans. They are made of metal and are comprised of two parts: the grooved tack section that pushes up through the denim and the top button that is then hammered on to the tack. The majority of jeans have a zipper fly with a stud button at the top to secure the pants.
Here are a variety of buttons: