A downdraft is a relative small scale downward current of air; often observed on the lee side of large objects restricting the smooth flow of the air or in precipitation areas in or near cumuliform clouds.
Microbursts are small-scale intense cold air downdrafts out of cumulus clouds or thunderstorm cells which, on reaching the surface, spread outward in all directions from the downdraft center.
This causes the presence of vertical and horizontal wind shear, especially at low altitude within some 1000ft of the ground.
Microbursts either occur as wet microburst carrying precipitation to the ground or as dry microburst descending from cumulonimbi or towering cumuli with a high cloud base (around 10000ft), typically in desert regions.
The strong downdraft is typically less than 1NM in diameter; the horizontal outflow can extend to approx. 4NM in diameter.
The downdrafts can be as strong as 8000ft/MIN. Horizontal winds speeds near the surface can reach up to 100KT.
They may be embedded in heavy rain associated with convective clouds or little or no precipitation. The main flight hazards of a microburst are:
• severe low level wind shear,
• the strong downdraft,
• severe turbulence at the gust front.
Other hazards found below thunderstorms may join the already critical situation, like heavy rain, hail, aquaplaning, lightning, and tornadoes.
Wind shear may be horizontal, vertical wind shear, and up and downdraft shears. A flight through a microburst lasts only for seconds.
The history of an aircraft encountering a microburst strongly depends on its path through the microburst and the phase of flight.
Source: ICAO , Wikipedia
Link to an explanatory article on Wikipedia here