How do NextTag, JellyFish, Google Adwords & Overture track orders?

NextTag.com is a service that aggregates product info from different vendors and offers comparison shopping. Essentially, vendors have to work to ensure that they offer the lowest prices for a product so that when a consumer compares prices among the same product types, their products get clicked.

When a consumer clicks on a product, they're taken to the vendors site and if they buy a product, NextTag gets a cut. In order to determine whether they came from a NextTag, a vendor needs to include the following snippet into the final page of the order process:




This snippet somehow allows NextTag to differentiate between normal orders and order explicitly referred by NextTag. For example, if you receive 10 orders today and 3 were referred by NextTag, the code snippet somehow knows this.

I want to do something like this but can't for the life of me figure out how they did it. I checked my cookies and there are no NextTag cookies set at all (sub-domain either). JellyFish.com, Google Adwords, & Overture all do something similar.

JellyFish:






Can anyone shed some insight on how this is done?