"Because that's where the Money is."

  With Affiliate programs you sell products that you do not have to stock. You can often make more than if they were your own products because you don't have shipping or storage expenses.  There are two ways to participate in these programs.  Many of these run through third party services such as Commission Junction.  With these you can join multiple programs and they keep track of all of them and combine your profits into one monthly check. The other way to join is of course directly through the particular program.
There are basically three different types of affiliate programs:
Pay-per-click - You get paid for every visitor clicks on your link.
Pay-per-lead - You get paid based on the number of actual leads that you send to the affilate site.
Pay-per-sale - This is the preferred method. This form of affiliate program only pays if a visitor from your site actually purchases the product. This method typically pays a percentage of the sale, and can pay as high as 60% of the purchase price. For more on Affiliate and Associate programs please visit Digital Income.net