Working and simple applications of PLL
Perhaps one of the most useful applications for a PLL is a frequency multiplier.

A PLL is a feedback loop. Feedback loops try to attain a steady state in which the input is equal to the output. By modifying the feedback mechanism, you can do a lot of interesting stuff...

Consider the following:



Let's say a frequency of 1MHz goes into the Pre Divider set to divide by 1. This 1MHz enters one of the Phase Detector inputs.
Now suppose the Feedback Divider is set to, say, M = 8.
This 1/8th of MHz enters the second Phase Detector input.

Because the loop is trying to reach a state in which the two inputs are equal, the VCO frequency will eventually have to rise to 8MHz. You just turned 1MHz into 8MHz using a couple of dividers and a VCO. 

You'll find a PLL multiplier in every ARM microcontroller. Using a set of various dividers for N and M you can generate many different frequencies from a single clock source.