Adsense is Google's contextual advertising service, which offers targeted income-generating advertisements to website owners. Google actually scans your web pages and displays relevant ads based on your content. For example, if you have a blog about legal advice, the ads displayed by Adsense would probably be for related sites like law firms.
Both text and image-based advertisements are available via Adsense, as well as a Google search box function. The colors of the text-based ads can be customized to match your site layout, and there are numerous ad sizes to choose from.
You earn revenue each time someone clicks on one of the Adsense advertisements. Most of the clicks I have seen earn less than 20 cents, so low traffic sites may not see much revenue. Certain keywords will pay higher, depending on the advertiser competition. Google pays out via monthly check or deposit. There is no cost to sign up for Adsense.