The Tourbillon is a device used in watches & clocks to counteract the effects of gravity on the accuracy of watches. The tourbillon was invented in 1795 by Abraham Louis Breuget, the great French watch engineer and designer. 

In the mid-1980s, after quartz watches had become the norm on most markets, there was a revival of interest in mechanical watches as a niche/luxury/ special-interest market. The tourbillon made a comeback; not as a practical enhancement, but simply an expression of the watchmaker's craft. Tourbillon's began to proliferate from Switzerland's elite watchmakers. Some have even attempted to tackle the 3-dimensional problem of making a toubillon useful in a wristwatch.

All watch movements are affected to a greater or lesser extent by gravity.  The least effect is in solid state movements, such as electronic digital watches, and the greatest effect is in mechanical movements, such as wind up or automatic movements. Breuget invented the tourbillon to ensure greater accuracy in mechanical watch movements by counteracting gravity. 

Tourbillons counteract gravity by rotating the balance wheel of a watch or clock through 360 degrees.

In Breuget's day all watches and clocks, except sundials, were mechanical.  With very few exceptions (there were a few wrist watches but these were very rare) watches were pocket watches which stayed in one position most of the time.  Imagine a pocket watch kept tucked upright in a man's vest pocket or a lady's handbag, the only time it changes position is when it's taken out by the owner to check the time.

The effect of gravity on a watch or clock kept in one position all the time, like a pocket watch, is particularly noticeable on the escapement, especially the balance which rocks back and forth. This can lead to inaccuracies.  Breuget's solution was the tourbillon.

Today, tourbillon movements are mostly found in wrist watches, which is pure vanity because most people keep their hands in near constant motion thereby providing the changes in position required to offset the effects of gravity on the watch movement.

In a tourbillon movement, usually just the balance wheel and escapement is rotated.  Typically the tourbillon is incorporated as a design feature of the watch or clock, mostly at the 6 O'Clock position where this delightful piece of minature engineering can be seen pursuing its never ending race around and around.  Occasionally, the entire movement is rotated, as in the mid-19th Century US made "poor man's tourbillon" a pocket watch in which the entire movement rotated through 360 degrees within its case.