Radar warning receiver
A radar warning receiver (RWR) detects radar signals that might be targeting the platform carrying the receiver, but does not give enough detail to identify or counter the radar. When used for military purposes, it is a tool of electronic intelligence. In general, the presence of a radar signal can be detected well before the radar can get a significant lock on the target (i.e., carrying the radar warning receiver).
RWRs differ in the signal type and frequency ranges of the signals they detect. The two basic types of signals are continuous wave and pulse doppler. Continuous wave radar is most commonly associated with semi-active radar homing missile midcourse and terminal guidance. Most tactical vehicles are otherwise threatened by pulse doppler search and midcourse guidance radars; active missile radar is also apt to be pulse doppler.
"Fuzz busters" and similar devices are intended to warn automobile drivers that they are approaching a police radar. More and more requirements are being found, such as defending civil aviation from terrorist threats. 
- Hooton, E.R. (December 2005), "Fighting an invisible threat: air warfare involves both visible and invisible battlefields and the latter becomes more demanding every year—having extended from combat aircraft to support aircraft and now down to the helicopters and even civil aviation", Armada International