Something that looks like sand, feels like sand, or is covered in sand is sandy. A sandy beach is lovely; a sandy bed is just uncomfortable.
If your dog tracks sand indoors and spreads it all over the place, you've got a sandy house. If your sister's hair is the brownish color of sand, you can describe it as sandy too. And if you spill your ingredients when you're making a cake, you might realize that sugar and flour both look a little sandy when they're on the floor. Sandy comes from the Old English sandig, "of the nature of sand."