Cookies are small files stored on the visitor’s computer that record information that is of interest to the merchant site. Despite concerns some people have, cookies are not dangerous, and can not be used to steal names, email addresses, phone or credit card number. Used with affiliate programs, cookies have two main functions:
a) to track what a visitor purchases and,
b) to track which affiliate made the referral

Be wary of programs which ONLY use cookies since they have many inherent limitations: they can be turned off by the visitor, they expire after a certain date or time, and they can be deleted off the visitors computer. Most programs use either unique URLs or affiliate ID numbers in conjunction with cookies to track properly. Cookies can then be used to give the affiliate credit at a later time of purchase, even if the visitor doesn’t click-through from the affiliate’s site again.