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What is swirl?
Swirl, in the sense of combustion, is an effect that is usually engineered to act on the air-fuel mixture as it enters the combustion chamber. By a variety of means, the mixture is made to 'rotate' around the spark plug inside the chamber. Swirl is considered to be a good thing because it leads to turbulence.
What is turbulence?
Swirl is a pretty smooth looking thing (when you see false-colour velocity plots, that is), but like all things smooth, it does not last. As the piston comes up during the compression stroke, it breaks the swirl, officially called decay. When swirl decays, you get a random motion of the mixture called turbulence. Turbulence is good because it removes the opportunity for the air fuel mixture to separate and produce inefficient and variable (across cycles) combustion. Turbulence, and the great mixing of the air and fuel droplets also leads of fast burning, clean burning, and hence efficient flames. In essence, turbulence leads to cleaner, more complete combustion. In the context of engines, turbulence is a great asset at lower revs. Making the mixture swirl takes time, and reduces the amount of mixture you can pump into the chamber. So when revs come up, engine builders prefer smoother flows and higher volumes. In essence, if you wanted a superb high-rev punch in the motor, you'd be working towards greater flow rather than turbulence.
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