What is the Doppler effect and how is it exploited in everyday life?

The Doppler effect describes the changes in a wave's frequency (or wavelength) as the source of the wave moves towards/ away from the observer (you). You have almost definitely experienced this phenomenon before - if you have ever listen to the siren from an ambulance or a fire engine as it drives past you then you would have noted that the frequency increases as it drives towards you and then decreases as it drives away. This is the Doppler effect in action. 

The Doppler effect has many uses in modern day life, such as in radar guns to measure the speed of vehicles and ultrasounds to measure the flow of blood around your body. The Doppler effect has also allowed us to greatly imporved our understanding of our universe: red-shift in light from distant galaxies provides evidence for the expansion of the universe.