A radar (Radio Detection And Ranging) is an instrument that operates by transmitting an electromagnetic radio wave toward some object. This wave is reflected by the object and some of the reflected wave is received back by the antenna of the radar. Receiving this wave means making a detection, or in other words, it means detecting that some object stands in the direction of propagation of the wave.
A radar can detect many properties of the target object: first of all, its distance, which is proportional to the delay time of the reception of the reflected signal.
Furthermore, a radar can detect many other properties of the target object by comparing the characteristics of the emitted and received signals. In fact, the beam initially emitted by a radar is somewhat like the beam of a laser light: it has a single frequency and it is coherent (meaning that the phase of the beam is the same across the entire wavefront). One of the greatest advantages is that the beam's characteristics (such as its frequency, its duration and its strength) are perfectly known and can be compared to the characteristics of the reflected signal. If, for example the frequency of the two signals are different, this means that the detected object is moving in the direction of the observer (this is due to a Doppler effect).