Before I go on…  If you have no idea what a browser cookie is I would suggest that you take a look at The Unofficial Cookie FAQ or browse on over to the Wikipedia article on the HTTP cookie.

Some people and anti-spyware software would have you believe that all browser cookies are bad (especially “tracking cookies”.)  When it comes to paid survey, incentive, GPT, and PTS sites (hereafter referred to as the “incentive site” for the sake of simplicity) cookies are your very best friend.

How do incentivized offers track?  Most use a combination of tracking cookies, the single pixel gif, a sub-id containing your site membership information, and your unique ip address.

1. When you click on an offer/survey the incentive site’s software will note your click.
2. You then will be sent to the offer’s url with a sub-id usually containing your incentive site user name appended to the end of the url.  Though is may seem instantaneous and direct on pretty much any connection other than dial-up, your browser is actually being sent on a wild goose chase of many different urls to get to your final destination (incentive sites use sponsor (or “CPA” / “affiliate”) networks as a source of their offers – those sponsor networks usually get their offers from other bigger sponsor networks.)  The only way to properly make sure that everyone the should be paid for your “lead” will be paid is to send you through a daisy-chain of successive URLs (each noting and crediting themselves your lead.)
3. Ok, you arrive at the survey/offer.  You fill out information, make a purchase, or perform whatever “action” is required to generate credit.  This is where the tracking cookie and single pixel gif come into play.  Once the action is performed then you are usually sent to a confirmation page that will place a cookie on your computer eventually reporting to the original incentive site that you have indeed completed the required action for credit.
4. Another reporting method is the mysterious single pixel gif.  This is a uniquely named image (picture) file that is so small (1 pixel by 1 pixel) that it is invisible to you that is placed on the confirmation page.  The gif is uniquely named so that it specifically and only refers to you specifically.  The offer’s web server knows that you have completed an action when a request is made to “show” that one pixel gif (known as the gif “firing” in the industry.)
5. If the offer site notices that you already have a cookie on your system from a previous session at that site (indicating that you are NOT a “unique lead”) then it may internally note this, allow you to continue, though ultimately report to the incentive site that you are not qualified for the incentive (ouch!)  The same goes for if you are recognized by your ip address or entered information such as your name, email address, etc.)  Your ip address also plays a part in identifying if your are within the required geographic location for that offer.
6. Once you are done viewing/completing the survey/offer, close browser window of leave the site, and end up back on the original incentive site then the tracking cookie reports that you have or have not qualified for your incentive.