From Bulbapedia, the community-driven Pokémon encyclopedia.
A sand tile is a special tile that can be found in the beach and desert areas of the Pokémon games. There are two types of sand tile: regular and deep.
Normal sand does not have any wild Pokémon, so it is very similar to a regular tile. Walking or running over sand leaves footprints, while cycling leaves a tire track; both disappear shortly after appearing.
Deep sand is a special type of sand that appears in Pokémon Ruby, Sapphire, Emerald, Black, and White. By entering patches of deep sand, wild Pokémon may appear and battle the player, making it the desert equivalent of tall grass. In Hoenn, it is found in the desert section of Route 111. In Unova, it is found on Route 4 and in the Desert Resort. In Generation V, it slows down the bicycle to speeds slower than the walking speed. In Alola, it is found in Haina Desert.
Differences between games
Certain moves have special effects when used in a battle on a sand tile.