It represents the percentage of users who took a decisive action.

"The two terminologies- cost per click and pay per click are often utilized interchangeably. They are also used as distinct terms sometimes. When these terminologies are used differently, CPC or cost per click represents the cost measurement on per-click basis for different contracts. They are however based on other aspects like click-throughs. PPC or pay per click, on the other hand represents the payment based on click-throughs. In a layman term, CPC represents the amount that advertiser has to pay a user every time the latter clicks on the formers ad. There is a separate pricing system in place for Google Ad Words."