Ghostery is a browser extension, a piece of software that extends the functionality of a web browser on a desktop computer. Ghostery monitors all the different web servers that are being called from a particular web page and matches them with a library of data collection tools (trackers). If it finds a match, Ghostery shows that tracker in the list in the pop-up information bubble or control panel. If you’ve configured Ghostery to block communication with one or more of these companies, it interrupts the call from leaving the browser.
This is different than opting-out or blocking cookies because those strategies still allow the browser to communicate with the web server (they just try to alter the communication or the response). When blocking is enabled, Ghostery never allows the communication in the first place.