Cookie Tracking: It is by far the most popular method of ad tracking. Cookies are small text files that are written to a user's browser whenever an affiliate's link is clicked. Thus the referral gets stored in cookie and is recognized at the order page. And affiliate is credited for the referred sales.

Not only they are simple to be read on any form, they are stored until commissions are rewarded to the referring affiliate. This way, affiliates are rewarded for the immediate purchase as well as the purchase made latter but within a specified period.

But remember that many people delete their cookies often, so you might be missing some affiliate sale due to that