Sand is a substance that looks like powder, and consists of extremely small pieces of stone. Some deserts and many beaches are made up of sand.

Sands are a large area of sand, for example a beach.

If you sand a wood or metal surface, you rub sandpaper over it in order to make it smooth or clean.

If you refer to the shifting sands of a situation, you mean that it changes so often that it is difficult to deal with.