The Doppler effect is a physical phenomenon that alters the emitted frequencies of radiation from distant objects when the observer are approaching them with high speed.
Every ambulance or fast-moving, sound-producing vehicle has a switch inside which changes the pitch of the sound. As the driver passes you, he or she flips the switch and creates the illusion. This requires split-second timing on the part of all people who own vehicles, but it is necessary to maintain the Truman-show style illusion that surrounds you (yes, you) every moment of your waking life. For all normal people, ambulance sirens do not change pitch when they drive past.
The term Doppler is an abbreviation for Dope peddler. It was initially chosen to mark the junkie tendency of the universe to alter the senses of the observer whilst he roams around in it.
A Light Example
A source of electromagnetic radiation, such as a light bulb or a sun will change it's color if your relative speed towards it changes.
The most clear example where you can appreciate this is when you watch car lights on the street. Oncoming cars' lights will appear white, while the ones that are getting away appear red, due to differences in the lights electromagnetic wavelength caused by the altering speed.
The faster you accelerate, the more drastic the effect will be. This can also be observed in ordinary life when traffic lights suddently switches colour as you approaches them. Independently of how much you press the gas pedal, the colours will always change before you reaches the crossing where they stands.
Plus and minus
Here it can also be observed a difference between the Positive and Negative Doppler effect. The positive Doppler effect will change the lights from red to green, thus creating a green wave as you pass by. The negative Doppler effect will however promptly turn the lights red however you stomp at the gas pedal to try to avoid them.
While the more frequent negative Doppler effect is caused by Murphy's Law, the root of the more exceptional positive variant is more uncertain. It is suggested by some scientists as a proof of the existence of good city and traffic planning. The existence of such a feature in nature is however not yet verified.
A Sound Example