The first clock used gravity pulled weights which moved gears, which moved the hands of the clock. The problem with this device was someone had to constantly reset the weights.
14th century clocks show the four key elements common to all clocks in subsequent centuries, at least up to the digital age:
-the power, supplied by a falling weight, later by a coiled spring
-the escapement, a periodic repetitive action that allows the power to escape in small bursts rather than drain away all at once
-the going train, a set of interlocking gear wheels that controls the speed of rotation of the wheels connected between the power supply and the indicators
-indicators, such as dials, hands, and bells