All modern clocks require four basic components. First, it requires a source of power, either mechanically or electronically. Second, it needs a sort of time base that will give the clock rhythm. Third, it requires a sort of gear system that will allow it different components of time (seconds, minutes, hours). Finally, a clock needs a way to actually display the time.
Revolutionized by the Dutch astronomer Christiaan Huygens in 1656, pendulum clocks were the first clock designed that maintained its accuracy. Several noticeable parts of the pendulum clock are the face of the clock, with its hour and minute hand, one or more weights, and the pendulum itself. Pendulums swing once per second in wall clocks, but it might swing up to twice per second in small cuckoo clocks, and in large grandfather clocks, the pendulum swings once every two seconds. The number of swings in the pendulum clock correlates with the lengths. The weight of the pendulum basically acts as an energy storage device, using potential energy as the weight swings back and forth while running the clock. The amount of time the pendulum takes to make a complete trip back and forth, the period, is related to the length of the pendulum and the gravity. Since gravity is constant, the only thing that affects the period of a pendulum is the length of the pendulum. Neither the amount of weight nor the length of the arc of the pendulum's swing matters.
A more complex use of the pendulum is the development of the escapement. In an escapement there is a gear with teeth and a pendulum attached to an item that is hooked to the gear. For each complete pendulum swing, one tooth of the gear escapes. It is this releasing of each tooth that causes the tick tock sounds clocks make. One thing to keep in mind is that pendulums will not swing forever. One additional job of the escapement gear is to give enough energy into the pendulum to overcome friction and allow it to keep swinging.