Ultrasonic range finders are unique sensors that use high-frequency sound waves to detect objects. The range finder is then able to measure the distance between the object and itself, and the robot can be programmed to react accordingly. This can be used to avoid bumping into walls or other robots, without the need to hit it, as would be needed with a bumper or limit switch. The frequency waves have a measuring range of 1.5 inches to 115 inches, which is over 3/4 the length of a standard VEX field.
The way that the ultrasonic range finder works is almost exactly the way that animals such as bats or dolphins use echolocation, or how a submarine uses sonar. The range finder sends out high-frequency waves that travel through the air. When the waves hit an object, such as a wall, the waves bounce backwards as an echo. The sensor then detects the returning sound waves and calculates the time it took for the sound to return. The longer the time it takes, the farther away the object. However, the ultrasonic range finder has troubles detecting softer objects, as the sound waves will be absorbed into the object and not return to the sensor.
VEX Sensors and How to Use Them 