Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. It includes several components, the most significant of which are wingtip vortices and jet-wash, the rapidly moving gases expelled from a jet engine.
This picture from a NASA study on wingtip vortices qualitatively illustrates the wake turbulence.
Wake vortices from a landing Airbus at Oakland International Airport interact with the sea as they descend to ground level.
XB-70 62-0207 following the midair collision on 8 June 1966.
Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift. The name is a misnomer because the cores of the vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes named trailing or lift-induced vortices because they also occur at points other than at the wing tips. Indeed, vorticity is trailed at any point on the wing where the lift varies span-wise ; it eventually rolls up into large vortices near the wingtip, at the edge of flap devices, or at other abrupt changes in wing planform.
Modern airliners often feature slender wings and wingtip devices
Vortices shed at the tips and from the leading-edge extensions of an F/A-18
Canada geese in V formation
A NASA study on wingtip vortices, illustrating the size of the vortices produced.