Clocks are typically made up of a face and hands, a gear system and a pendulum or quartz crystal circuit.
Face and Arms
The visible parts of the clock are the clock face and the hands. The clock face is the circular plate, usually with numbers or Roman numerals inscribed on it, upon which the clock's hands rotate. There are usually three hands: an hour hand, a minute hand and a second hand. The rotation of the hands directly corresponds with the movement of the wheels.
Wheels
Clocks and watches typically consist of three wheels: the center wheel, the third wheel and the escape wheel. The center wheel rotates once per hour, the third wheel rotates once per minute and the escape wheel regulates how fast the mainspring unwinds.
Mainspring
In mechanical clocks and watches, time is kept by the release of potential energy contained in a wound-up spring called the mainspring.
Pendulum
Some clocks use a pendulum system to keep time. A pendulum is a weight that hangs from a pulley mechanism in a clock and swings at an exact rate of once per second.
Quartz
Clocks that use quartz to keep time contain a precisely shaped quartz crystal that resonates at an exact natural frequency. The crystal is connected to an oscillating circuit, causing the circuit to resonate at this same frequency. This determines the rate at which time passes in an electrical clock.