Pearl sugar consists of large opaque white grains that look much like the large-grain salt you often see on giant soft pretzels sold in malls and ballparks. This variety of sugar withstands high heat without melting, so it makes a wonderful garnish for sprinkling on cookies, muffins, and scones before baking. It can be purchased in a variety of colors.
Sanding sugar is clear and sparkly, unlike the opaque pearl sugar, and its extra-large crystals also will not melt in the oven. Sanding sugar is available in many colors.
Both varieties can be found in specialty grocery stores, the cake-decorating section of many crafts stores, and at online sources, such as kingarthurflour.com.