CPC stands for “cost per click” and is a term generally used with search and text advertising campaigns where you only pay for the number of clicks you actually get on your ads no matter how many times they see the ad before they click on it.
Example: If the cost of your CPC ad is $1.00 and it is seen 10,000 times but nobody clicks on it, you pay nothing. But if 10 people click on your ad then you will pay $10.00.
For the truly technical at heart here is a link to the definition for Actual CPC from the Google AdWords Glossary. Because there are plenty more complicated nuances to bidding and calculating your CPC campaign to learn about if you are curious to find out more.