Sandpaper

Sandpaper is basically sheets of stuff coated with some kind of abrasive (particles of aluminum oxide, silicon carbide, garnet, etc.). It's used for smoothing out the surfaces and edges of an object. They come in a variety of backings (paper, cloth, plastic, etc.), but the most common one you'll find is paper. Different abrasives are suited to different jobs. For example, alumnium oxide is great for woodworking while silicon carbide is best for hard materials, like plastic or metal.

You'll often see a number associated with sandpaper. That's the grit of the sandpaper. The lower the grit, the coarser the paper. The higher the grit, the finer the paper. You usually start with a low grit (to get rid of big or deep scratches) and gradually switch to higher (or finer) grits. For example, if you were sanding a piece of wood with some surface scratches, you'd start with 100 grit, then switch to 120 grit, then 150 grit, and finish with 180 grit.