Google: Google AdWords

Google AdWords is Google's advertising system in which advertisers bid on certain keywords in order for their clickable ads to appear in Google's search results. The following infographic will help you understand how AdWords works, detailing the AdWords auction, bidding process and explaining important factors like Quality Score and cost-per-click.

Reference:
What Is Google AdWords? How the AdWords Auction Works
http://www.wordstream.com/articles/what-is-google-adwords