Refer to figure.
WIND SHEAR is a sudden change in wind direction and/or speed over a very short distance. Wind shear can be broken up into:

• Horizontal wind shear - A change in the wind over a horizontal distance
• Vertical wind shear - A change in speed and/or direction with change of height
• Updrafts and downdrafts - Air traveling vertically up or down

Windshear can cause rapid changes in:

• Airspeed: eg. A decreasing headwind / increasing tailwind will result in a slower airspeed.
• Angle of attack: eg. A sudden updraft will change the relative air flow and so also the angle of attack, possibly resulting in a stall.

Wind shear may be associated with a thunderstorm, a low-level temperature inversion, a jet stream or a frontal zone.