Roff is the man to explain it in detail but as I understand it, a tourbillon is intended to keep a watch accurate, no matter what angle the watch is at.

Without wishing to stir up a hornet's nest, IMHO a (genuine) tourbillon is purely a (very expensive) decorative addition to ultra high-end watches and will probably not be any more accurate than a COSC certified watch.

And a $20 Casio will be more accurate than a $200,000 tourbillon!

http://en.wikipedia.org/wiki/Tourbillon

Wikipedia isn't bad on this.

The tourbillon concept is from the time of pocket watches when the watch was worn vertical in a pocket. That meant that the balance was always running in a vertical plane which meant that it's accuracy was impacted by gravity. The tourbillon is designed to contantly rotate the escapement through 360 degrees to offset the effect of gravity. It worked well (in theory) in the days of pocket watches but today it really has dubious benefits because watches aren't kept in a consistent plane.

There are much better ways of ensuring accuracy than a tourbillon for sure, and the only 'tourbillon like' (it's not a true torubillon) complication that really has the potential to improve accuracy is Zenith's Zero G as that keeps the balance in a horizontal plane regardless of the position of the watch.