A flow meter is, in simple terms, a device that measures the flow of something.  There are different ways of doing this, and therefore, different types of flow meters.

• Mechanical flow meters.  These use mechanical means to measure the flow.  For instance, a setup where the flow turns a wheel, or is constricted through a pipe and the pressure measured.
• Optical flow meters.  An optical flow meter uses some type of light and sensor to measure flow.  For instance, in the case of a laser pointed at a sensor, when liquid flows in front of it the light scatters.
• Magnetic flow meters.  A magnetic flow meter uses magnetism to measure the amount of flow, but for obvious reasons these do need the pipe that the fluid flows through to be insulated and non-magnetized.
• Ultrasonic flow meters.  An ultrasonic water flowmeter is similar in some respects to the optical flow meter, except that it uses sound to measure flow instead of light: The meter sends out a pulse, which it then uses to measure flow.

Flow meters have come a long ways, and are much more precise and advanced than they used to be.  For example, you can get a liquid flow calibrator that gives accurate digital readings for greater ease of use.  Companies such as Sierra Instruments are on the leading edge of this technology.