Pay Per Click (PPC) is a term used to describe a particular advertising model. Cost Per Click (CPC) is simply a metric that is used to measure the what clicks cost within the PPC advertising model.
* PPC = Type of advertising program
* CPC = A metric used in PPC Campaigns
I can't resist the temptation to use the movie analogy
If you are going to see a movie at a Drive-in theatre you might ask if it's pay-per-car or pay-per-person. If it's pay-per-person, then you might ask what is the cost-per-person.
PPC is the equivilent of describing the payment method and CPC is the equivilent of describing the cost within that payment method. Just replace the word "person" with "click" in the above analogy.