The abrasive particles are glued to a sheet of paper, cloth, etc. to form sandpaper. My old woodwork teacher used to insist that sandpaper was called ‘glasspaper’, because the abrasive used back then was glass powder. Today neither glass nor sand (silica) are found on sandpaper. Instead, the familiar substances silicon carbide, aluminium oxide, and diamond are used. As there is only one layer of abrasive the sandpaper has a limited life — the abrasive particles being broken or detached from the backing relatively quickly. On the other hand, sandpaper is cheap and flexible. An effective and inexpensive (though shortlived) whetstone can be made by taping sandpaper to a flat base.
A Stanley 60½ plane cutter in an Eclipse 36 sharpening jig