Vortex Generators?

I know what vortex generators are for, but how do they improve a plane's performance? Are these really necessary, and why don't all aircraft have them?

Vortex Generators?myspace pics

Vortex generators are added to the leading edge of a swept wing in order to maintain steady airflow over the control surfaces at the rear of the wing.

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 aircraft carry vane vortex generators from time of manufacture, but there are also after-market suppliers who sell VG kits to improve the STOL performance of some light aircraft.

Air jet vortex generators work on a different principle. They direct a jet of air into the boundary layer, thereby re-energising it.

Vortex generators are also being used in automotive vehicles. In one form they are used as in aircraft to influence the boundary layer of air flow primarily for drag reduction. In another form they are installed in the engine's air intake hose. Manufacturers claim that the vortex generator creates a swirling motion within the air intake pipe, and within the combustion chamber causing improved burning of the fuel, increasing horsepower and fuel efficiency.

Vortex Generators?pimp myspace myspace.com

They drastically improve low-speed performance by lowering the stall speed. They energize the air flowing over the wing to delay the point of boundary layer separation at high angles of attack.
You may know what vortex generators ARE, but you obviously don't really know what they ARE FOR or you wouldn't be asking how they improve a plane's performance (sorry for the slap-down, but you asked for it)

A vortex generator creates a small tip vortex that introduces fast moving free air into the relatively slow-moving boundary layer to re-energize it. The "boundary layer" is the thin layer of air next to the airfoil that is slowed through friction with the skin and normally weakens and thickens as it moves aft along the airfoil surface. A weak boundary layer separates from the airfoil sooner at higher angles of attack and also reduces the effectiveness of trailing edge control surfaces. By introducing energetic air into the boundary layer the vortex generators enhance control effectiveness and help delay flow separation that leads to an aerodynamic stall.

Not all planes have them because vortex generators are a relatively recent innovation and they can be an expensive retrofit on older aircraft. The benefit does not always outweigh the cost, or they would be more prevalent. On top of that, they interact with different wing designs in varying ways, sometimes not effectively, so a costly aerodymic study must be done (usually modeled in a wind tunnel) to determine size, shape, and placement before they can be approved for use on a particular design. In place of vortex generators, some planes (particularly stock Cessna and Piper singles) have small triangular "turbulator strips" placed on the leading edge of the wings to accomplish the same purpose. Although they are cheaper and easier to install, they aren't quite as effective.
Some of the people ahead of me already covered most of your question, but this link explains vortex generators in a very detailed manner, and also has diagrams, to help you understand. It also demonstrates the nacelle vortex generator.

http://old.smartcockpit.com/operations/V...
Basically and briefly and in general terms: vortex generators, as you know, direct or re-direct air flow. The directed or re-directed air flow improves stability with respect to the flight control airforils thereby improving either stability or lowering wing load stall speed, or both, and sometimes they are even utilized on jet engine air intakes; the wing on a jet isn't the only thing can can "stall" (lose adequate lift), a jet engine can stall-out if the angle of attack (angle of air flow as compared to the attitude of the engine) becomes to great too fast.

Hope this helps.