Radar is a frequently used technology, that's application can be seen in such products as grocery store doors, police speed detectors, and is used by planes to detect aircraft and other objects. The primary use of radar is to detect and gather information of moving and stationary objects, but has many more applications than just that.
Radar relies on the concepts of Doppler shift and echo location. Each is a common phenomenon that you experience each day but are probably unaware of. Doppler shift occurs when sound is reflected, generated, or projected from a moving object. A good example of Doppler shift would be if a car blaring it's horn went by you at a high speed, the sound would get louder and have a high tone as the vehicle got closer and fainter with a lower tone as the vehicle passed and got further away. It is possible to combine Doppler shift and echo to get a precise calculation on how fast that car is moving. If you were able to send out loud sounds toward the car, some of the waves would bounce off the moving car and echo back towards you. However, these sound waves will be compressed, causing them to have a higher pitch that the original sound that was generated. By measuring the pitch of this echo we are able to determine how fast the car is moving. .
The use of sound to locate objects and use it for tracking is the idea behind sonar, or "sound radar". This kind of tracking, which is also known as "echo location", is sufficient for submarines and boats due to how well sound travels through water. Unfortunately there are some problems using sound outside of an aquatic environment: sound doesn't travel too far, (a mile at most) and a faint echo would be received making it difficult to make accurate readings from the results. Another problem would be presented with the audible noise that sonar produces making it dissicult, if not impossible, to mask it's use. Radar on the other hand, uses this same concept to transmit and receive its signal but instead of sound, radar uses radio waves.