Google's Adsense program is designed to match advertisements to the content on a publisher's web site. In theory, AdSense will choose relevant ads, provided through Google's AdWords program, to display to your visitors. If a visitor is interested in an AdSense ad they will click on it, giving the AdSense publisher credit for their click.
Google shares a portion of their AdWords income with AdSense publishers who's sites have generated clicks. It is Google's goals with AdSense to make advertisements on publisher sites relevant and useful to web site visitors, and to expand the available ad coverage for AdWords advertisers.