There are three different Adsense "products" that you can promote in exchange for revenue. These include Adsense For Content, Adsense For Search and Referrals.

Adsense For Content

This is the most common used and exists in the form of different shaped ads which you will have no doubt seen scattered over the web. These ads are relevant to the content of your website and you earn revenue every time one of your users clicks on them. For more read different ad formats.

Adsense For Search

This product allows you to place a Google search box on your website, allowing your visitors to search either your website (if your pages are indexed) or Google's search engine database. Each search by your users will generate ads related to that search term and you earn revenue for every ad click by your users.

Referrals

The Referrals product is a little different as you are not earning revnue through contextually matching ads on a per click basis but are recommending useful products and / or services to your users. When one of you visitors subscribes to the product or service you receive a commission. Referrals are much more like affiliate programs and they currently include Google Adsense Subscription, Google Adwords Subscription, Firefox Download and the Picasa Software Application.