I'm trying to write up a simple explaination to a client about the difference between Max CPC and what you end up paying for a click...anyone have a good short explanation for a client that has no knowledge of PPC?
MAX CPC is simply the most that you are willing to pay for a click; so Google will never charge higher then that. In 99% of cases, you will always be charged lower then your MAX CPC; since it depends on several factors such as competition and your own ad performance. The more competition there is, or the worse your campaign performance, the closer to your MAX CPC you will pay.
mariya is quite right. But, I'd like to add to this one fact from a live experiment i conducted: I had a $0.05 max cpc for one ad group on both search and content. But, I was charged $0.06 per click (for over 600 clicks). Although, it baffled me. But, this soft of thing does happen.