A shelf is a flat horizontal plane made of a strong material such as wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 or bamboo

Bamboo

Bamboo is a group of perennial evergreens in the true grass family Poaceae, subfamily Bambusoideae, tribe Bambuseae. Giant bamboos are the largest members of the grass family....

 or steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 which is used in a home

Home

A home is a place of residence or refuge. When it refers to a building, it is usually a place in which an individual or a family can rest and store personal property. Most modern-day households contain sanitary facilities and a means of preparing food. Animals have their own homes as well, either...

, business

Business

A business is an organization engaged in the trade of goods, services, or both to consumers. Businesses are predominant in capitalist economies, where most of them are privately owned and administered to earn profit to increase the wealth of their owners. Businesses may also be not-for-profit...

, store

Store

Store may refer to:*a retail store*a place where things are stored, e.g. a ship's paint store*expendables released from an aircraft, such as ordnance or countermeasures*Štore, a town and a municipality in eastern Slovenia...

 or elsewhere to hold items of value that are being displayed, stored or offered for sale. It is raised off the ground and usually anchored/supported on its shorter length sides by brackets

Bracket (architecture)

A bracket is an architectural member made of wood, stone, or metal that overhangs a wall to support or carry weight. It may also support a statue, the spring of an arch, a beam, or a shelf. Brackets are often in the form of scrolls, and can be carved, cast, or molded. They can be entirely...

. It can also be held up by columns or pillars.

A shelf can be attached to a wall

Wall

A wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...

 or other vertical surface, be suspended from a ceiling, be a part of a free-standing frame unit, or it can be part of a piece of furniture such as a cabinet

Cabinet (furniture)

A cabinet is usually a box-shaped piece of furniture with doors or drawers for storing miscellaneous items. Some cabinets stand alone while others are built into a wall or are attached to it like a medicine cabinet. Cabinets are typically made of wood or, now increasingly, of synthetic...

, bookcase

Bookcase

A bookcase, or bookshelf, is a piece of furniture, almost always with horizontal shelves, used to store books. A bookcase consists of a unit including two or more shelves which may not all be used to contain books or other printed materials. Shelves may be fixed or adjustable to different positions...

, entertainment center

Entertainment center

A home entertainment center is a piece of furniture seen in many homes in North America, which houses major electronic items, such as a television set, a VCR and/or DVD player, stereo components , and cable or satellite television receivers...

, some headboards

Headboard (furniture)

The headboard is a piece of furniture that attaches to the head of a bed. In previous times and in older less well insulated buildings they serve to isolate sleepers from drafts and cold walls...

, and so on. Usually two to six shelves make up a unit, each shelf being attached perpendicularly to the vertical or diagonal supports and positioned parallel one above the other. Free-standing shelves can be accessible from either one or both longer length sides. A shelf with a hidden internal bracket is termed a floating shelf

Floating shelf

A floating shelf is a form of shelf designed so as to avoid showing any unsightly brackets, appearing as though it is "floating" on the wall.- Design and Use :...

. Shelves meant for holding lighter-weight objects can be made of glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 or plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

. The length of the shelf is based upon the space limitations of its siting and the amount of weight which it will be expected to hold. The vertical distance between the shelves is based upon the space limitations of the unit's siting and the height of the objects; adjustable shelving systems allow the vertical distance to be altered. The front edge of the shelf under the object(s) held might be used to display the name, product number, pricing, and other information about the object(s). The unit can be fixed or be some form of mobile shelving

Mobile shelving

Mobile shelving or roller racking systems are specialised storage units designed to offer increased capacity for a given footprint over traditional filing units.- Mobile Storage Systems :...

. The most heavy duty shelving is pallet racking.

There are different senses of the word shelf based on its storage application. In a quality sense, top shelf usually refers to where the best quality wine or liquor are kept in a specialty liquor store or bar: the best product is kept at the top behind the bar or cash register

Cash register

A cash register or till is a mechanical or electronic device for calculating and recording sales transactions, and an attached cash drawer for storing cash...

 for display until someone needs it. In a marketing

Marketing

Marketing is the process used to determine what products or services may be of interest to customers, and the strategy to use in sales, communications and business development. It generates the strategy that underlies sales techniques, business communication, and business developments...

 sense, a product which is on the shelves is displayed for consumers

Consumer

Consumer is a broad label for any individuals or households that use goods generated within the economy. The concept of a consumer occurs in different contexts, so that the usage and significance of the term may vary.-Economics and marketing:...

 and is available for sale. In a political

Politics

Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

sense, the term shelve may mean that an issue which was being considered actively was no longer being considered actively, that is, an agenda item was shelved meaning that it is not at the forefront of the political agenda while it hasn't been rejected entirely.