Flightradar24 shows live airplane traffic from different parts around the world. The technique to receive flight information from aircraft is called ADS-B. That means the Flightradar24 can only show information about aircraft equipped with an ADS-B transponder. Today about 60% (about 30% in USA and about 70% in Europe) of the passenger aircraft and only a small amount of military and private aircraft have an ADS-B transponder.
Flightradar24 has a network of about 500 ADS-B receivers around the world that receives plane and flight information from aircraft with ADS-B transponders and sends this information to a server, and then displays this information on a map on Flightradar24. Only aircraft with an ADS-B transponder, within the coverage area of the 500 receivers are visible.
Link to Flightradar24 website: http://www.flightradar24.com