Aircraft carry transponders that when interogated by ground based radar reply and give the information requested. ADSB
http://en.wikipedia.org/wiki/Automatic_dependent_surveillance-broadcast gives off among other information the position of the aircraft. The Radarbox and other receivers use this information to show where the aircraft is on the map. Non ADSB equipped aircraft dont give positional information and cannot be displayed but you know that they are around as their signal is picked up and they are shown on the grid. Most but not all airline traffic is ADSB equipped. Most smaller aircraft, military aircraft are not ADSB equipped but it is becoming more common on even the samllest aircraft.
MLAT uses information from boxes to triangulate the apporioximate position and Planepotter and FR24 use this information to display the aircrafts position when there are sufficient users to allow triangulation. Radarboxes are not able to provide the output required to allow triangulation
In the USA, the FAA provides data, 5 minute delayed which can be used in browser based applications such as RB24 to display its delayed position.
Alan