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I saw a vortex generator mentioned in an aircraft listing.

What is a vortex generator?

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To be more general, a vortex generator is a small angled plate installed on an outer surface of an aerodynamic body. The angle of the plate causes the air to swirl, creating a vortex behind it. This effect allows the air flow to remain "attached" to the surface even at points where the flow without a vortex would separate from the surface.

One of the most common uses of vortex generators is on aircraft wings forward of the ailerons. When the aircraft wing stalls, the flow detaches from the wings. This means that the flow will detach before it reaches the ailerons, making them ineffective. The use of vortex generators helps the ailerons to provide control even if the rest of the wing is stalled.

Flow with and without vortex generators in stall
Source

Of course, these also add drag, even when the wing is not stalling, so they are only added where needed.

They are also used in other places, such as the engine, as in Farhan's comment above, and also on the tail of the 737.

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I will try to explain the physical mechanism, adding extra information compared to previous answers.

As mentioned before, the vortex generator is usually taking the shape of a plate. What this plate is doing is generating an small vortex. This vortex is basically a region where the flow is rotating around its axis. Basically this plate extracts energy from the flow generating this rotation on the flow.

Ok, this rotational flow has got energy and, if properly oriented interacts with the boundary layer over the wing providing further energy to it. With that extra energy the boundary layer is more resistant to separation, that means, that you can increase the angle of attack further and obtain a higher lift coefficient for the same wing.

Now comes the why?. A vortex generator is always creating drag, always but it might reduce the fuel consumption of the airplane. How?

Think about overall design of the airplane, for a given flying condition the amount of lift generated can be obtained big lift coefficient times wing surface.

Imagine now, that we are designing the airplane and we have found that the condition that is dimensioning the needed size of the wing is, for example, the maximum runway length at take off, in order to reduce it you need to generate more lift at an specific speed. For being able to accomplish that you can either increase the wing surface or increase the maximum lift you can obtain from the wing, and you can accomplish that by introducing vortex generators smartly designed for that condition.

A trade off is later on made between increasing the wing surface (with the impact in drag and weight) and including the vortex generators. So, although adding those vortex generators are adding drag compared to a clean wing, it might be the case that you are actually reducing fuel consumption because the other alternative is adding more wing surface and the optimum solution became the vortex generators.

There are several location of vortex generators, some of the on the vertical plane improving rudder efficiency, others in the upper surface of the wing, others over the engine affecting the slat during take-off or landing.

Another use of vortex is after designing the airplane as quick fix to solve “unexpected issues”.

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They're little winglet type things that you install on the surface of the wings to encourage laminar flow of air and to discourage stalls. Think of them as "cheap insurance for when you're not monitoring your airspeed very well".

Edit for future readers: Should be attached flow, not laminar flow. See below comments for explanation.

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    $\begingroup$ uh-o - replace "laminar" with "attached" and the answer looks good. $\endgroup$
    – Peter Kämpf
    Commented Apr 7, 2015 at 4:46
  • $\begingroup$ Okay, teach me. I thought there were two types of flow in a fluid (air in this case) -- laminar and turbulent. I thought air transitions from laminar flow across the surface of a wing to turbulent (stalled) flow, and vortex generators somehow cause that transition to be delayed. $\endgroup$
    – Calphool
    Commented Apr 7, 2015 at 14:21
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    $\begingroup$ @Calphool Wikipedia has a good cursory overview, as does the image in fooot's answer. Basically it's the change between "attached" and "detached" flow (the separation point) that matters for delaying the stall, and attached flow can be laminar or turbulent. Attached turbulent boundary layers resist being detached from the surface (wing) more than a laminar boundary layer does, so basically vortex generators delay the flow separation by making the air more turbulent in a particular way such that it's less likely to separate from the wing. $\endgroup$
    – voretaq7
    Commented Apr 7, 2015 at 19:50
  • $\begingroup$ @Calphool: voretaq7 beat me to the right answer. It is basically about mixing flow near to the wing surface (which is slowed down due to friction) with flow at some distance which can then re-invigorate the boundary layer (flow near to the wing surface). If the flow near the wing is slowed down to a standstill, flow separates and the wing loses some of its effectivity. If you ask a new question, I will explain it in more detail than is possible in a comment. $\endgroup$
    – Peter Kämpf
    Commented Apr 7, 2015 at 20:11
  • $\begingroup$ @Calphool: Laminar and turbulent are the two types of boundary layer, a thin, almost microscopic, sheet of air close to the surface. Attached and separated flow is another concept (and the one which vortex generators address) which concerns the macroscopic flow pattern around bodies. Vortex generators will make sure that the boundary layer behind them is turbulent, so that is the reason for my first remark. $\endgroup$
    – Peter Kämpf
    Commented Apr 7, 2015 at 20:16

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