The wikipedia article[1] of Ghostery gives a quick & fast overview of what they do. MIT Technology review posted[2]:

> Evidon sells two main services based on the data it collects. One allows website operators to see which tracking code, from which companies, is active on their site and how it affects the speed with which its pages load. The other provides ad companies with figures on how common the tracking code from different companies is around the Web.

[1] https://en.wikipedia.org/wiki/Ghostery#History_and_use

[2] http://m.technologyreview.com/news/516156/a-popular-ad-block...