For those who don't know, ad extensions are features that can be added to your AdWords ads. Some of these include Ad Sitelinks, Product Extensions, Video Extensions, Location Extensions, and Click to Call Phone Extensions.
Upon logging in to AdWords, you will probably not see this tab unless you have enabled ad extensions in at least one of your active campaigns. When clicking on the tab, you will be able to see the stats for any of your campaign ad extensions. You can also decide which data you want to see. You can see everything or filter out deleted ad extensions, or just filter the results by extension type.
Here is a screenshot right from the AdWords blog: