Clock towers are a specific type of structure that house a turret clock and have one or more clock faces on the upper exterior walls. Many clock towers are freestanding structures but they can also adjoin or be located on top of another building. Some other buildings also have clock faces on their exterior but these structures serve other main functions.
Clock towers are a common sight in many parts of the world with some being iconic buildings. One example is the Elizabeth Tower in London (usually called "Big Ben", although strictly this name belongs only to the bell inside the tower).