The phenomenon of apparent change in frequency and wavelength of a wave when source and observer move relative to each other is called Doppler effect. In our daily life, the sound of an ambulance becomes louder when it approaches us and becomes dimmer when it passes away from us. This is the general example of Doppler effect. The mathematical phenomenon of Doppler effect was given by Christian Doppler 1842.
We can consider Doppler effect in wave motion for sound waves as well as electromagnetic waves. The microwave Doppler effect is used to check the speed of the vehicles in radar gun as well as tracking satellite.