The wet bulb temperature is the temperature of a thermometer that is placed in a wetted medium and has air blown across it. Combined with the dry bulb temperature, the wetbulb temperature can be used to determine the percent humidity and the dewpoint. It should be noted that at 100% humidity, the dry bulb and wet bulb temperature are the same.
Example:
The wet bulb temperature is why you might feel cold in a pool room after getting out of the pool. The evaporating water, makes the surface temperature of your skin cooler than it would be when dry (assuming the pool room is at less than 100% humidity)