what is clock in computer science

This was taken from http://en.allexperts.com/q/Computer-Science-3197/Clock.htm Answering the question of the question of clock generator in a computer.

Clock generators are used on PC motherboards to generate the various system clocks. If you have a ‘dead' motherboard, chances are good that this part of the motherboard still works. Spotting the clock chip is easy; there are only a few manufacturers, the chip has a crystal nearby, it's usually a SSOP-type package and series-terminating SMT resistor arrays surround the chip.

The chips are usually a W83196s-14/28509300/9030CA chip with a blip of a bubby that is actually the crystal. Then you see two terminating wires near it with a resistor.

Once you have a chip, Google for then read the datasheet for the device. The Winbond W83196S-14 (don't confuse it with the A version) is described as a “100 MHz CLOCK FOR BX CHIPSET”, with 12 discrete frequency pair selections, 10 outputs that can be active or in a high impedance state, a spread spectrum capability, 50% duty cycle outputs, and an I2C bus for control. Required power supplies are 2.5V and a 3.3V, and 3.3V-only operation is possible. It has a default power-on state so the chip has active outputs even without a controller. (You can just apply power and check the outputs to make sure the chip is working.)

If you are asking about clock speed (of a CPU) here is a good answer from wikipedia

The clock rate is the rate in cycles per second (measured in hertz) for the frequency of the clock in any synchronous circuit, such as a central processing unit (CPU). For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry (AD Converter) into a corresponding square wave pulse [typically] for digital electronics applications. In this context the use of the word, speed (physical movement), should not be confused with frequency or its corresponding clock rate.