What is a downburst and why can it cause airplanes to crash?

DownburstA downburst is created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. There may or may not be rain reaching the ground. It is a threat to aviation since it can suddenly change relative air speed of an airplane as it encounters a localized headwinds or tailwinds, suddenly increasing or decreasing lift.