The terms pay-per-click (PPC) and cost-per-click (CPC) are sometimes used interchangeably, sometimes as distinct terms. When used as distinct terms, PPC indicates payment based on click-throughs, while CPC indicates measurement of cost on a per-click basis for contracts not based on click-throughs.
For example, consider a campaign where payment is based on impressions, not clicks. Impressions are sold for $10 CPM with a click-through rate (CTR) of 2%.
1000 impressions x 2% CTR = 20 click-throughs
$10 CPM / 20 click-throughs = $.50 per click