Definition
The wet-bulb temperature is the temperature to which a parcel of air at a constant pressure cools through the evaporation of water into it. At this temperature, the parcel becomes saturated.
In other words, take a parcel of air, which is not at saturation. Then, at constant pressure (with no vertical motion), evaporate water into the parcel. Evaporative cooling will occur until the parcel reaches saturation. The wet-bulb temperature is reached when the air parcel achieves saturation.
The wet-bulb temperature will always fall between the dewpoint and the temperature, unless the air is saturated. At saturation, the temperature, dewpoint, and wet-bulb temperature are equal.
In the real atmosphere Tw often provides a good estimate of what the surface temperature will become after the onset of precipitation and once conditions become saturated.