Sand. One has it and the other doesn't.
Sanded Grout contains about 30-40% of a fine grain sand, which decreases the shrinkage rate of grout as it cures. It also reinforces the grout to allow it to be used in joints up to 1-2" in width. Most sanded grout is used in widths of 1/8" up to 1/2" wide.
On the other side of the Sanded would be the Unsanded grout. Unsanded contains absolutely zero percent of sand. And in it's place is around 30-40% of a material called Calcium Carbonate, which is essentially finely ground white marble dust. This material has less impact on shrinkage and reinforcement, and should be used in grout joints that are less than 1/8" wide. Unsanded should also be used with softer tiling materials such as Glass, Marble, Travertine, Encaustic Tiles, and some Natural Stones.
We offer both types of grout. If you need help deciding which one would be best for your application, then please send us an email at email@example.com.