The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding.
The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc. We are most familiar with the Doppler effect because of our experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source.
The Doppler effect is used in many technologies that benefit people. But it can have a negative impact, as well. For example, sonic booms, which are caused by supersonic aircraft, can cause objectionable sounds and vibrations on the ground, which is why supersonic airplanes are not allowed to fly over populated areas. Sonic booms are directly related to the Doppler effect. They occur when airplanes, flying at the speed of sound or higher, actually fly faster than the sound waves they are producing. All of the waves bunch up behind the craft, in an extremely small space. When the bunched-up waves reach an observer, they are "heard" all at once -- as a resounding boom.
The Air Force and NASA are experimenting with several inventions that help mitigate sonic booms. One such invention is a spike extending from the nose of the airplane. This spike essentially lengthens the plane and distributes the waves over a greater distance. This reduces the boom experienced by an observer on the ground.