A gust front, also known as an outflow boundary, is created when air that has been cooled by a thunderstorm sinks rapidly. When this air gets to the ground it spreads out, creating the gust front that moves through an area ahead of a thunderstorm.
As precipitation falls, it can evaporate in the relatively dry air of the lower atmosphere. As this happens, the air cools and becomes more dense. This cool, dense air sinks toward the ground as part of the downdraft and creates the gust front once it hits the ground and begins spreading out. Because of this, the gust front separates the warm, humid air that is feeding the thunderstorm in the updraft with the cool, dry air that is in the downdraft of the thunderstorm.
Shelf clouds and roll clouds can form along gust fronts, and the winds generated can be very intense and damaging. If the winds are especially damaging, it is known as a downburst.