A practice that underhandedly deposits cookies from merchants onto users computers when the user has never visited the merchant through the affiliate's link and potentially may not have even the affiliate's website. Cookie stuffing is done with the intent of stuffing as many cookies as possible onto as many user computers as possible in the hopes that they eventually come accross the merchant website and make a purchase. The larger and broader the merchant, the more likely that is to occur (think Amazon). Cookie stuffing is heavily looked down upon by legitimate affiliates and most merchants ban affiliates using cookie stuffing in their affiliate agreements.
And what about joining another company's affiliate program? It's all about extra revenue. Think about your customers' needs: What other products or services would interest your site visitors? Join those affiliate programs. Affiliate programs can increase your sales with no upfront cost to you. It just takes a little time to plan your strategy and select the partners that will have the greatest impact on your business.