Here is how pay per click programs work. As a advertiser, you bid the price of the keywords you are willing to pay for visitors to come in. As a publisher, you receive the amount of bidded price less the charges by the program owner that supplies you with this facility. In short:

Advertisers : Bid on a specific keyword that he is willing to pay when a visitor clicks on to be directed to a specific site
Publishers : Receive profit less the amount charged by program owner when someone clicks on an advertisement from advertiser. The advertisement would be located at the publisher’s site

Here are a few giant sites for PPC:

THE BIG ONES
Google Adwords and Adsense
Yahoo (Formerly Overture)
Miva (Formerly Findwhat & Espotting)
Adbrite

THE SMALLER ONES
Search123
enHance
SearchFeed
GoClick
7Search
Findit-Quick
Looksmart
Kanoodle

For Google, Adwords is for advertisers and Adsense is for publishers. Google will split the earnings with the publishers which a specific advertisement is clicked. However, Google never reveals the percentage of the sharing with his publishers.

For example, let’s say you are the publisher and you site is about dogs. You would have to put a code snippet into your site as a publisher. Once you have done that, Google will place all the advertisement form the advertisers about dogs (dog care, types of dogs, dogs for sale etc). Let’s say a particular advertiser is paying $1.00 per click and Google shares 50% of the commission with you. You would be receiving $0.50 for that particular click. If you want to know more about Adsense, you can click here