Your Cost Per Click, often referred to as CPC, is the cost that you pay when you receive a click on your advertisement. As it pertains to Google, your Actual CPC is determined by several factors including Quality Score and Ad Rank.
Maximum CPC is the highest price that you are willing to pay for a click on your advertisement. You can set your maximum CPC’s at the either keyword level or at the ad group level. “The AdWords Discounter automatically reduces this amount so that the actual CPC you are charged is just one cent more than the minimum necessary to keep your position on the page.” This means essentially, that you will pay, based on a couple of factors, one cent more than the person below you.
Your Actual CPC is the amount you’ll actually pay for a click on your ad — no more than the maximum bid you specify for your ad group, keyword or placement. The AdWords Discounter automatically gives you the lowest possible price in order for you to maintain your ad’s position.