What is the difference between wet bulb and dry bulb temperature?

Dry bulb temperature is the actual air temperature measured by a standard thermometer. Wet bulb temperature is measured by a thermometer with its bulb wrapped in a wet wick; evaporative cooling lowers the reading below dry bulb temperature. The difference between them indicates the air's humidity - a larger difference means drier air. These measurements are used with psychrometric charts for humidity calculations in drying and air conditioning.