A shelf is a flat surface for holding things.
Shelves can be attached to the wall, be part of or set on top of other furniture or freestanding, as in a bookcase. They can also be built into a structure -- in cubbies or like a mantle shelf. Shelves are often found in closets, cabinets, on some desks and other storage spots.
Shelves are useful to store or display books, clothes, knickknacks, collectables or other objects.