Definition: a common advertising model where the publisher (site owner) gets paid a set amount for every click generated on their web properties. Simply stated, it is the amount paid to have an advertisement clicked once.
There are a number of PPC (Pay Per Click) networks, the most popular being Google AdWords. This is also the hardest one to get approved for as well. If you are having trouble with your Adsense approval you should check out this article: Google AdSense Approval in 8 Steps
Pay per click is also referred to as: cost per click (CPC) although this isn't as common.
PPC is most commonly served through contextual advertising. Learn more about contextual advertising and why it works so well.
PPC is also known as: CPC Cost Per Click.
Some online business models operate on the primary income stream of PPC ads.