An Affiliate program basically promotes another person's products or services for a commission, which is usually a percentage of the resulting sale.   

The official definition as listed in: http://www.webmaster-affiliates.net/dictionary.php  is :
A mutually beneficial relationship between a website and a company. The company seeks to generate more web traffic, sell more products, or seek out potential customers through leads. The website seeks to make a profit by selling ad revenue on the site. Affiliate programs come in many shapes and forms.

One of the easiest ways to find companies with an affiliate program is to sign up for Linkshare and other companies that handle the affiliate programs for numerous companies. 

Another approach is to search the web for the type of sites that you would like to promote. Pick sites that match your niche and turn your information site into an earning site!!