A photographic technique developed in 1864. It makes use of the change in the refractive index that accompanies changes in density. Therefore it is able to depict shock waves (eg in wind channel tests) or the flow of warm gas or liquid (eg above a flame). "Schliere" by the way is German for "streak".
Update:
Due to popular demand and rampant downvoting, I'll try and elaborate a bit. And I hope I'll get it right!
The thing is that the speed of light depends on the refractive index of the medium. Therefore (this may not be exactly obvious, but Huygens and wave optics say so) light gets refracted when the medium changes. The difference in speed can also cause the effect of interference, but that's for another day (see also Why does oil on water look like a rainbow).
In our case, we make a photo of lamp in a mirror.
In front of the mirror is our object, eg a hot cup of coffee. But now comes the trick. We cover the lens of our camera for the most part so that light from our source can only enter if it was refracted by some density fluctuation (some light may also enter "conventionally" so that we will be able to see the cup). This gives us a nice black and white picture of the swirls rising from our hot coffee. Using color filters we could even do some more tricks.
A full (and rather technical) explanation can be found at
http://www.rit.edu/~andpph/text-schlieren.html
Update:
iansmith has a nice tutorial with pictures on his page at http://www.ian.org/Schlieren/HowTo.html