Google Adsense

Google one of the biggest Search, Web, and Advertising companies on the Internet has a program called AdSense. AdSense is a free Blog and Website Advertising program where a Site Owner can cut and past an html Script Advertisement into their site.
AdSense differs from most other Free Advertising Services, in the fact that Adsense is highly customizable. Site owners can choose from a variety of Ad Formats, shapes, sizes, Appearance, Picture Ads, Text Ads, Referral Ads, Video Ads, and Mobile Ads.

The 5 types of advertisements:

AdSense for Content - allows Site Owners to choose Ads that through a Page Crawling Spider determine appropriate Ad content. The AdSense for Content Ads come in Ad Units, Link Units, and can be customized to be Image Only or Text Only. site owners can choose Competitive Ad Filter options so the Ads don't directly compete with their Business.

AdSense for Search- a highly Customizable Search Box that displays Ads with in the search queries. when people hit the Advertisements in the Search Queries the Site Owners gain revenue from this program.

AdSense for Referral- you can choose specific referral ads or categories. referral ads offer all of the customizable ad options available in the content ads with the added text ads of which you get three. though there is some speculation that the text ads can negatively affect the SEO (Search Engine Optimization) of your site nevertheless these are the most powerful money earning ads that AdSense offers.

AdSense Video Units- offer a three size customizable YouTube video player that the site owner receives revenue on ad clicks in the ad on the screen and the ad above the screen.

AdSense for Mobile - offer script ads for your mobile web sites

Peter Chen's blogger tips site is a staple for basic questions surrounding the AdSense
Program

most web publishers who use AdSense regularly check out the inside AdSense Blog for up to date Information on the dynamic and continually improving program