See "How Affiliate Marketing Works" in the "Did You Know..." section of Webopedia.
Also see Refer-It.com, a directory of referral & affiliate programs for e-commerce, featuring information on over 6,500 affiliate programs.

ECommerce-Guide Affiliate Week Profile: Amazon Associates
Launched in 1996, the Amazon Associates affiliate program is the granddaddy of all e-commerce affiliate programs. Amazon Associates has evolved over the years and currently offers a number of different opportunities for affiliates to profit from.

ECommerce-Guide Affiliate Week Profile: eBay Affiliates
If you're looking for a way to earn income from eBay without having to actually sell anything yourself, the eBay affiliate program is something to consider.

ECommerce-Guide Affiliate Week Profile: Google AdSense
Making money with Google is easily done. With a few basic guidelines, it's almost a sure bet your site can generate incremental revenue.

Refer-It.com
The Refer-It database lists thousands of programs offered by e-commerce vendors that share revenue with Website owners.

Webopedia's 'Did You Know... How Affiliate Marketing Works?'
Affiliate marketing is the term used to describe a revenue-sharing plan where an online automated marketing program lets Webmasters place an advertiser's banner ads or buttons on their own Web site. Webmasters will receive a referral fee or commission from conversions when a customer has clicked the affiliate link and performs the desired action, such as make a purchase or opt-in for downloads or newsletters on the advertiser's site.