Glossary
CPC
The price of one click on your ad. As a buying model: you pay per click, not per impression. As a metric: CPC = spend / clicks. The starting point of every campaign's economics.
Also called: cost per click, PPC.
In a CPC source you pay only when someone clicks, so the network takes the risk of showing ads nobody wants. In a CPM source you pay per impression and CPC is what your creatives earn you: a better click-through rate is a lower CPC.
CPC is only meaningful next to conversion rate and payout: a $0.02 click that converts at 0.1% costs $20 per lead; a $1 click at 10% costs $10.
PPC (pay per click) describes the same model from the advertiser's side, and is also the name of search advertising as a discipline.