Conversion tracking is a tool to help you measure conversions, and ultimately help you identify how effective your Ad Exchange ads are for you.
To begin using conversion tracking, you must first put the conversion tracking code on to your site. Follow the complete instructions to start getting metrics for your conversions.
Once the code is correctly applied, conversion tracking will place a cookie on a user's computer or mobile phone when he or she views or clicks on one of your Ad Exchange ads, unless a user has opted-out of cookies in their internet browser user preferences. Then, if the user reaches one of your conversion pages, the cookie is connected to your web page. When a match is made, Google records a successful conversion for you.
Please note that the cookie Google adds to a user's computer or mobile device when he or she views or clicks an ad expires in approximately 30 days. This measure, and the fact that Google uses separate servers for conversion tracking and search results, protects the user's privacy.