Wake Vortex Turbulence
All aircraft generate vortices at the wing tips as a consequence of producing lift. The heavier the aircraft and the slower it is flying, the stronger the vortex. Among other factors, the size of the vortex is proportional to the span of the aircraft which generates it.
At low altitudes, vortices generally persist for as long as 80 seconds, but in very light or calm wind conditions, they can last for up to two and a half minutes. Once formed, vortices continue to descend until they decay (or reach the ground). Decay is usually rapid and occurs more quickly in windy conditions. Cross-winds can carry a vortex away from the flight path of the aircraft.
Wake vortices are also a hazard at en-route altitudes:
For further informaiton read the lead article about Wake Vortex Turbulence.