As you browse the Web, Privacy Badger looks at any third party domains that are loaded on a given site and determines whether or not they appear to be tracking you (e.g. by setting cookies that could be used for tracking, or fingerprinting your browser). If the same third party domain appears to be tracking you on three or more different websites, Privacy Badger will conclude that the third party domain is a tracker and block future connections to it.
Doesn’t work with Safari but I’d recommend everyone reading this site on Chrome / Firefox install it. EFF is the most stand-up group to create such a product.