@ Heathrow Director:
The reason lies behind the fact that radar is electromagnetic energy being transmitted, or in other words, it is a radio signal being transmitted and the reflections (Primary Radar) or the responses (Secondary Radar) are received. Put simply, anyone with the correct equipment is also able to 'listen' to the radar transmissions of others, much like someone with a scanner is able to listen to ATC chatter by tuning their receiver to the correct frequency.
By building databases of various radar transmitters, military users are able to discern the type of radar being employed. I've heard that in some cases military users are even able to distinguish between different aircraft/ships using the same radar equipment, due to small differences between sets.
In any case, Wikipedia (out of many sources) goes into far more detail than I have here.
General Radar Article
ESM Article (Interception of signals for military purposes)