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.
A WebSDR is a Software-Defined Radio receiver connected to the internet, allowing many listeners to listen and tune it simultaneously. SDR technology makes it possible that all listeners tune independently, and thus listen to different signals; this is in contrast to the many classical receivers that are already available via the internet.