When you have finished this page, try the Humidity Quiz. Humidity is moisture in the air. There are several measures of humidity. One is absolute humidity which is the greatest amount of water vapor the air could hold at a certain temperature.
Relative humidity is how much actual water vapor is in the air at a certain temperature.
Thus humidity is expressed in terms of percents. Humidity is measure with a hygrometer. The rate of evaporation goes down as the humidity in the air increases - there is not enough room in the air to take more water vapor through the process of evaporation.