Affiliate marketing is a way for a blogger or web author to make money by referring visitors or customers to a business's website.  A blogger can earn a commission if a visitor simply subscribes to the site or if they make a purchase.  Commission can either be a percentage of the sale or a flat fee.  Most businesses belong to an affiliate network company which serves as the middle man and handles all of the transactions including making sure you get paid.  Here are the top 4 affiliate network companies.

Commission Junction is the affiliate network company for companies like Coupons.com,  Toys R Us, etc.  Once you select the company you would like to join, they will review your site and let you know if you have been approved.  The Commission Junction website has all of the available banners and links for each company.

LinkShare.com works just like Commission Junction.  Some of the companies associated with Linkshare are 1-800-Flowers, iTunes, and South Beach Diet.  Banners and text links are also available.

Amazon.com Associates Program
Amazon.com Associates program allows bloggers to advertise Amazon products on their blog or website and earn a commission.  Signing up is quick and easy.  Amazon offers many different types of banners, widgets and astores.   

Share a Sale
Shareasale.com is an affiliate network which manages affiliates such as websites and online stores. Signing up with Share a  Sale is also very easy and sometimes instant affiliate approval.

Clickbank
Clickbank.com is an affiliate network that mostly has ebooks and digital products.  A blogger can simply sign up and then visit the marketplace to select ebooks to advertise on their blog or website.