Let's imagine you contact an ad network and they say you can pay per impression (every time the ad is displayed) or pay per click (every time the ad is clicked). This is generally known as CPM advertising (pay per 1,000 impressions) and PPC advertising (pay per click).
If you choose PPC, you'll need to pay for every click. But how much will you pay? That is where CPC (cost per click) comes in. Generally, the CPC will change depending on factors such as which countries are being targeted, what ad keywords you're using, etc.