In synchronous digital electronics, such as most computers, a clock signal is a signal used to coordinate the actions of two or more circuits. A clock signal will oscillate between a high and a low state normally with a 50% duty cycle. The circuits using the clock signal for synchronization may become active at either the rising or falling edge of the clock signal.
Most integrated circuits of sufficient complexity require a clock signal in order to synchronize different parts of the chip and to account for gate delays. As chips get more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes more and more difficult. The preeminent example of such complex chips are microprocessors, the central part of modern computers.
The clock frequency of a microcomputers CPU is very often used as a primary measure of the computers performance. CPU clock frequency is traditionally measured in MHz, but improved technology has moved this into the GHz range. Note that many factors affect computer performance, the clock frequency is only one of them.