The clock in a microprocessor serves to coordinate the operations of the different parts of a microprocessor.
A clock is basically an oscillator which generates a series of pulses. The pulses usually are square pulses. The output of the oscillator goes from low to high and then from high to low. The clock signal is generated by a clock generator.
The is usually designed using a quartz crystal. The clock can also be made using LC and RC circuits. Quartz offers stability and consistency(no drift) and is therefore preferred.
The circuits of the microprocessor can be designed to become active either during the rising edge or the falling edge.
The Speed of the clock will be in MHz or in GHz.