Google Adsense

Web Page:
What is it: Contextual text and/or image advertising for website publishers.
Revenue Type: Ads appearing may be pay per click or pay per view ads. In theory the ads that generate the most income will appear.

Pros: Easy to sign up, Easy to set up, Easy to use. Advertisers can target your site specifically.

Cons: You need a website that meets their terms of service to get started. You need a decent amount of traffic to see a decent amount of revenue. Google keeps a lot of information about rates, "smart" pricing and click fraud under their hats.

Overall: If you have a website with decent content and traffic you can probably benefit from having Google Adsense ads. Focused information based sites are often the best at generating revenue. If you plan to add Google Adsense to your sites, do the site design with them in mind from the beginning. Placement and color choice make a huge difference for revenue generation.