An RPM sensor measures rotation speed, usually of a crankshaft in a vehicle engine or on wheels in vehicles that move on a rail. They are more commonly called "tachometers."
Tachometers can be found in all types of vehicles, including aircraft. This information is usually conveyed to the operator through a gauge. In vehicles with manual shifting, a tachometer is necessary to know when to change gears.
Trains and other vehicles on rails use tachometers to measure wheel rotations. There are a number of applications for this such as automatically calibrating wheel diameter and tracking the total distance traveled.
Tachometers can also be found in audio tape recorders that use cassettes.