A key is a device which is used to open a lock. It usually consists of a specially-shaped shaft of metal, with teeth and/or grooves which fit the shape of the lock and can open the correct lock by (usually) being turned in the lock housing. A wider grip is found at the top of the key to facilitate the turn.
Most people in modern society use keys on a daily basis, to secure their home, their vehicle, or to access their workplace, among other uses. Those that use a number of keys will usually place them on a ring, often with other items such as key fobs.