Allows us to see in the dark and a long way away.
Fitted to most aircraft, ships and countries to detect nasty people, before they twat them back to obscurity.
Also on some missiles and guns to guide them onto targets.
Radar is now used for meteorology as well as detecting the fuzzywuzzys.
It works by sending out a radio wave and receiving the reflection of the signal, which gives a fairly accurate fix. Can be defeated by stealth or by putting a bloody great radar seeking missile into your ground station.