[K Reloaded]

A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus locked on) the frequency of an input signal. PLLs are used for frequency control. They can be configured as frequency multipliers, dividers, demodulators, tracking generators or clock recovery circuits.

On a computer motherboard a PLL is used to generate several frequencies that are required for proper operation:

http://www.techpowerup.com/articles/overclocking/28