Difference between average CPC and Maximum CPC?

Average CPC - also referred to as Avg. CPC:
This is literally the average amount you pay each time someone clicks your ad. The Avg. CPC you see in your account is determined by totalling the cost of all clicks and dividing that total by the number of clicks.

                                                                     (vs)

Maximum CPC - also referred to as Max CPC:

Maximum cost-per-click (CPC) is set by you, the advertiser, and is the highest amount that you are willing to pay for a click on your ad. You can set a maximum CPC at the keyword or ad group level. If you're interested in a deeper look at Max CPC, you'll find more information.