When you signup as an affiliate with an affiliate merchant like ClickBank, they will provide you with an unique affiliate link for each seller(publisher). When you promote(send traffic) using these affiliate links, a sale(conversion) will result in a sale commission for you.
Such a link would take the form:
http://youaffiliateid.publisherid.hop.clickbank.net
When you send visitors to this link, ClickBank creates an affiliate cookie which it then sends to the visitor’s browser. The browser then creates a cookie file that contains your affiliate id.
Usually, the affiliate cookie’s filename can be identified as it normally contains the name of the site, such as fabian@clickbank.net. The cookie might contain other information, such as an expiration date or a count of how often you visit the site. But most of the cookie’s contents are unintelligible to anyone who doesn’t already know what pieces of information are being stored there.
As this visitor has now been "cookied", if he goes on to make a purchase ClickBank will read the cookie on his machine and attribute the sale to you.
The visitor does not have to purchase immediately. As long as the purchase is made before the cookie’s expiry date (60 days for ClickBank) you will receive the credit.
While we are on the topic of affiliate promotions, now would be a good time to explore how you can setup a new account at ClickBank.