A vortex generator is an aerodynamic surface, consisting of a small vane that creates a vortex.  Vortex generators can be found on many devices, but the term is most often used in aircraft design.
Vortex generators are likely to be found on the external surfaces of vehicles where flow separation is a potential problem because vortex generators delay flow separation. On aircraft they are installed on the leading edge of a wing in order to maintain steady airflow over the control surfaces at the rear of the wing. They are typically rectangular or triangular, tall enough to protrude above the boundary layer, and run in spanwise lines near the thickest part of the wing. They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.
A vortex generator creates a tip vortex which draws energetic, rapidly-moving air from outside the slow-moving boundary layer into contact with the aircraft skin. The boundary layer normally thickens as it moves along the aircraft surface, reducing the effectiveness of trailing-edge control surfaces; vortex generators can be used to remedy this problem, among others, by re-energizing the boundary layer. 
Vortex generators delay flow separation and aerodynamic stalling; they improve the effectiveness of control surfaces (e.g Embraer 170 and Symphony SA-160); and, for swept-wing transonic designs, they alleviate potential shock-stall problems (e.g. Harrier, Blackburn Buccaneer, Gloster Javelin).
Many of the vortex generator kits available for light twin-engine airplanes bring with them the added benefit of an increase in maximum takeoff weight. This is paradoxical because installation of vortex generators does not increase the strength of the wing.
The maximum takeoff weight of a twin-engine airplane is determined by structural requirements and one-engine climb performance requirements. For many light twin-engine airplanes the one-engine climb performance requirements determine a lower maximum weight than the structural requirements. Consequently, anything that can be done to improve the one-engine-inoperative climb performance will bring about an increase in maximum takeoff weight.
“All multiengine airplanes having a stalling speed
Installation of vortex generators can usually bring about a slight reduction in stalling speed of an airplane and therefore reduce the required one-engine-inoperative climb performance. The reduced requirement for climb performance allows an increase in maximum takeoff weight, at least up to the maximum weight allowed by structural requirements.
An increase in maximum weight allowed by structural requirements can usually be achieved by specifying a maximum zero fuel weight or, if a maximum zero fuel weight is already specified as one of the airplane’s limitations, by specifying a new lower maximum zero fuel weight.
None of the requirements applicable to single-engine airplanes as their maximum takeoff weight is dependent on stalling speed so there is no opportunity for vortex generators on these airplanes to bring about an increase in maximum weight.
Similarly, after 1991 the airworthiness certification requirements in the USA have specified the one-engine-inoperative climb requirement as a gradient independent of stalling speed, so there is no opportunity for vortex generators to increase the maximum takeoff weight of multi-engine airplanes whose certification basis is FAR 23 at amendment 23-42 or later.
Because most light twin engined aircraft landing weights are determined by structural considerations and not stall speed, most VG kits only increase the take-off weight available and not the landing weight. In these cases increasing the landing weight requires either structural modifications or else re-testing the aircraft to demonstrate that the certification requirements are still met at the higher landing weight.