An anemometer is a tool that measures wind speed. There are several types of anemometers available but the the most common used for HVAC vent readings are vane and hot-wire anemometers.
Vane Anemometers - use propellers to take an analog or digital reading of the wind speed based on the velocity of the propellers rotation.
Hot-wire Anemometers - use an electrically charged fine metal wire to measure airflow. As air passes over the wire, it is cooled. A lower wire temperature results in a lower resistance to electricity allowing the anemometer to calculate airflow. (featured in video)