Wall clocks come in many varieties, but they generally belong to one of two categories: mechanical clocks and battery or electric clocks. Of these two groups, mechanical clocks have significantly more operating parts. The power source of mechanical clocks are weights on chains that are periodically raised or springs that are periodically wound.
The power source of mechanical clocks go through a series of gears in order to reduce their energy to smaller amounts. The energy is then modified by an oscillating device that has a consistent frequency that ticks off seconds at a time. Generally, these are the pendulums or balance wheels. A pendulum uses a precise weight at the end of a rod to achieve a consistent frequency. A balance wheel uses a disc weighted on the perimeter and a torsion spring to first spin clockwise and then counter-clockwise to achieve a similar frequency. All of this achieves the motion and time-keeping of a mechanical clock.
In contrast, a modern electric or battery clock has fewer parts. Generally, a quartz crystal at the center of the clock is measured by an electronic oscillator for vibrations, and these vibrations are turned into the frequency that regulates the time-keeping pieces of the clock.