According to the National Weather Service, a microburst is a localized column of air (downdraft) within a thunderstorm and is usually less than or equal to 2.5 miles in diameter. Microburst can cause extensive damage and have winds upwards of 100 mph or more. Keep in mind that an EF-0 tornado has winds between 65-85 mph and an EF-1 tornado has winds between 86-110 mph.
What causes a microburst? A developing thunderstorm has an updraft that keeps water droplets and hail suspended in the upper portions of the thunderstorm. The updraft can be quite strong and keep the core of water droplets and hail for quite a bit of time. As the water droplets and hail grow in size the updraft struggles to keep everything suspended. Once the water droplets and hail become too heavy they plummet to earth. The rain and hail drag down the air with it. As it hits the ground it spreads in all directions. The area the microburst hits the ground experiences the highest winds and greatest damage.
Example: Picture your sink faucet. Water leaves the faucet and plummets and hits the sink and the water travels in all directions. The water falling drags the air down with it. This is why there are gusty winds with strong showers and thunderstorms. The faster the water the faster the winds will be.