The most common way tracking is done via passing information via appending the click destinations.
When you provide a landing page for an ad it might initially look like this:
http://www.SampleAdvertiser.com/products/product5.html
The landing page is then appended with what is called the UTM.
Below is a landing page URL with click tracking parameters from GA.
http://www.SampleAdvertiser.com/products/product5.html?utm_source=Adhesive&utm_medium=Display&utm_campaign=Adhesive%20Coop
The Red part is the URL of the page a person is directed to after clicking on an ad.
The Blue part specifies the what type of information is being collected when a user click on the ad.
The Bold Black parts are client specified and can be anything. These are the labels for the green parts that the client is tracking in their reporting.