Phase Locked Loop. It is the primary circuit in the processor that sets the overall clock. PLL's are used anywhere that a device is clocked against a reference where the resulting clock is higher than the reference clock.
Refer to this post: http://www.xtremesystems.org/forums/showpost.php?p=2824649&postcount=6
Wiki Page: http://en.wikipedia.org/wiki/Phase-locked_loop