Both sanded and unsanded grout are used to fill gaps between tiles. Sanded grout should be used for gaps larger than 1/8” in wide. Sanded grout is more durable than unsanded grout because it resists cracks from shrinkage. However, sanded grout can potentially damage and/or scratch smoother tiles.
Unsanded grout should be used for gaps that are less than 1/8” in wide. Unsanded grout is preferable for highly polished stones, like marble. Unsanded grout also provides a better hold on vertical surfaces in comparison to sanded grout.