Hook-and-loop sandpaper has a backing made of a material similar to Velcro. It has hooks and loops that are used to attach the sandpaper to a machine such as a random orbital sander or disc sander. This makes it very simple to change the sandpaper.
Some varieties of sanders are made to be used with hook-and-loop sandpaper, while some can be adapted for use with hook-and-loop by using a converter disc. You can buy hook-and-loop sandpaper in a variety of shapes, most commonly discs, although there are also star-shaped and triangular pieces available. You can buy sheets or rolls of hook-and-loop sandpaper, which you can cut to fit your requirements. All grades of sandpaper, from coarse to superfine, are available in hook-and-loop form to be used with power sanders.