When customers visit your website and you have a link provided by the merchant that is clicked on, there will be a cookie stored in their computer.  Depending on the merchant that cookie will last anywhere from 24 hours to permanent (until it is removed). If the customer purchases anything that has your affiliate cookie in it you will get a commission.

Cookies were invented so web sites could keep track of things individual browsers did and to be able to recognize the return of specific browsers.

In essence, the web site puts together an information packet for the browser to keep (the cookie). The browser gives the cookie back when it returns — so long as the cookie has not expired.

The information the web site gives to the browser (the cookie):

1. Identifies the browser. Only this individual browser has this individual cookie.
2. Contains information the web site can use. As examples, the cookie might contain the number of pages visited, the time of the last visit, or complete shopping cart contents.