A third party who provides affiliate program management to a merchant. Affiliate networks provide the technology for tracking affiliate efforts, ensure that sales are properly tracked, commissions are paid to affiliates, handle reporting for both the merchant and individual affiliates and help expose the merchant to potential affiliates for their products and services. You can find a list of the more mainstream affiliate networks here.
Translation: Amazon won’t let you join their affiliate program if your site is involved in anything illegal, deceptive, violent, or hateful. But even if this doesn’t describe your site (and I hope it doesn’t!), keep in mind that sites of an “adult” nature may not be allowed either. Also, even if your site is totally family friendly, if it’s focused on kids it may not be allowed, as Amazon says it won’t accept sites directed toward kids who aren’t teenagers yet.
You should also make sure you aren't competing with your own affiliates for eyeballs. Any marketing channels you're using, such as search engines, content sites or e-mail lists, should be off limits to your affiliates. Put marketing restrictions into your affiliate agreement and notify partners immediately. It's your program--you set the rules. Or, if you prefer, you can let your affiliates run the majority of your internet marketing.