Humidity sensors are used measure humidity. These sensors are used in process industries where the humidity of the atmosphere or a closed surface is a critical parameter. It is also widely used in medical instruments in respiratory devices. Humidity sensors have a special polymer which absorbs the water vapour. The absorption depends on the availability of water vapour in the surround air. This, in turn, is related to the humidity of the atmosphere. 
Thus, the sensor is able to measure the humidity.
Humidity sensors are classified into two broad types
Capacitive Sensors and Resistive Sensors
In capacitive sensors the polymer which absorbs the moisture is sandwiched between two metallic plates. When the humidity increases, the capacitance increases.
In resistance based, humidity sensors, a thin film of metal is printed on a board in a zig zag pattern. A film made of the moisture-absorbing polymer is applied over the board. When the moisture increases, the resistivity of the board decreases. This is measured by the sensor.