A police radar gun operates by transmitting radio waves at certain frequencies which reflect off objects (your vehicle) and are then picked up by the radar gun's receiving section. When radar waves reflect off a moving target, a measurable frequency shift occurs. The radar unit converts this shift into kilometres per hour to determine the target's speed.
Two basic types of radar are used - stationary and moving. Stationary radar must be used from a static site, typically a patrol car parked along side the road. With a single antenna moving radar, an officer can clock approaching vehicles while driving on patrol. Moving radar with two antennae - one facing forward, the other aimed out the back of the patrol car - can also clock vehicles even after they have passed by, headed in the opposite direction.