Flightradar24 shows live air traffic from around the world. The primary technology they use to receive flight information is called automatic dependent surveillance-broadcast (ADS-B). 
Flightradar24 has a network of about 500 ADS-B receivers across the world that receives plane and flight information from aircraft  and sends this information to a flightradar24 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.
Flightradar24 only have coverage in areas where someone has installed an ADS-B receiver and feeds his/her data into Flightradar24. If there are no connected ABS-B receivers in an area, then there is no coverage in that area.

In addition to ADS-B data, we also display data from the Federal Aviation Administration (FAA). This data provides full coverage of the airspace above the United States and Canada.
Flightradar24 covers about 90% of Europe. There is also some coverage in USA, Canada, Australia, Brazil, Middle East, Japan ,India and rest of world

Flightradar24 relies on volunteers around the world for the majority of our coverage. Find out how you can contribute.
Basic data flow in Flightradar24

Aircraft gets its location from a GPS navigation source (satellite)=>ADS-B unit on aircraft transmits signal containing about the location (and much and more)=>ADS-B signal is picked up by a receiver connected to Flightradar24 =>Receiver feeds data to Flightradar24

Today, roughly 60% of all passenger aircraft (70% in Europe, 30% in the US) are equipped with an ADS-B transponder. This percentage is steadily increasing as ADS-B is set to replace radar as the primary surveillance method for controlling aircraft.