A footstool is a piece of furniture

Furniture

Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...

, the purpose of which is to support one's feet. There are two main types of footstools, which can be loosely categorized into two categories, those designed for comfort and those designed for function.

Footstools for resting feet

This type of footstool is used to provide comfort to a person seated, for example, in a chair

Chair

A chair is a stable, raised surface used to sit on, commonly for use by one person. Chairs are most often supported by four legs and have a back; however, a chair can have three legs or could have a different shape depending on the criteria of the chair specifications. A chair without a back or...

 or sofa

Couch

A couch, also called a sofa, is an item of furniture designed to seat more than one person, and providing support for the back and arms. Typically, it will have an armrest on either side. In homes couches are normally found in the family room, living room, den or the lounge...

. It is typically a short, wide, four-legged stool

Chair

A chair is a stable, raised surface used to sit on, commonly for use by one person. Chairs are most often supported by four legs and have a back; however, a chair can have three legs or could have a different shape depending on the criteria of the chair specifications. A chair without a back or...

 with a padded top, upholstered

Upholstery

Upholstery is the work of providing furniture, especially seats, with padding, springs, webbing, and fabric or leather covers. The word upholstery comes from the Middle English word upholder, which referred to a tradesman who held up his goods. The term is equally applicable to domestic,...

 in a fabric or animal hide

Hides

A hide is an animal skin treated for human use. Hides include leather from cattle and other livestock animals, alligator skins, snake skins for shoes and fashion accessories and furs from wild cats, mink and bears. In some areas, leather is produced on a domestic or small industrial scale, but most...

, such as leather. This type of footstool is also a type of ottoman

Ottoman (furniture)

An ottoman is a piece of furniture consisting of a padded, upholstered seat or bench having neither back nor arms, often used as a stool or footstool, or in some cases as an improvised coffee table. Ottomans are often sold as coordinating furniture with armchairs or gliders.An ottoman can also be...

. It allows the seated person to rest their feet upon it, supporting their legs at a mostly horizontal level, thus giving rise to use of the term footrest, for this item.

Footstools for standing on

This type of footstool, also known simply as a stool, is used as a short stepladder. The footstool is used by placing it on a flat surface and standing on it to extend one's reach. For example, one design is known as an elephant's foot, which is a short stool which can be used to reach an object from a high shelf. It is designed to be more safe than standing on on a chair or desk.

Footstools under the feet while sitting

This type of footstool supports a person's (usually a child's) feet that do not reach the floor when seated. The footstool is placed under the feet of a sitting person so that the person's feet may rest comfortably on it. An example is the type of piano

Piano

The piano is a musical instrument played by means of a keyboard. It is one of the most popular instruments in the world. Widely used in classical and jazz music for solo performances, ensemble use, chamber music and accompaniment, the piano is also very popular as an aid to composing and rehearsal...

 footstool used in conjunction with a piano bench. It is also used to make the blood circulation of your body flow more freely than sitting down.