Fleur de sel (pronounced "flure-de-SELL") is a rare and expensive form of sea salt that is harvested in parts of France. The name fleur de sel is from the French for "flower of salt."
Most of the salt that's used in cooking is either dug out of the ground, in much the same way coal or other minerals are extracted, or via a technique similar to fracking, where fresh water is pumped into the ground above a salt deposit.
The salt dissolves in the water, and the salt-saturated is then pumped out and evaporated on large heated pans.
Both these these methods are comparatively simple, albeit performed on a large scale.
Fleur de sel is produced by a process that is considerably more complicated, which involves collecting the thin layer of salt that rises to the surface of shallow pools of seawater along the coast of France, mainly but not exclusively in the Brittany region. Similar high-end sea salts are produced in parts of Spain, Portugal and the U.K.
Because it forms naturally under certain very limited weather conditions, and because it must be skimmed off by hand, fleur de sel is, by any estimation, an extremely expensive product — $30 per pound or even more.
This high cost means that fleur de sel shouldn't be used as an ordinary seasoning. Instead, think of fleur de sel as almost a garnish or condiment. A few crystals of fleur de sel sprinkled over a dish right before serving add a burst of flavor, visual appeal and even texture.
Here's a 4.4 oz container of fleur de sel (about 125 grams) harvested in coastal parts of the south of France, near where the Rhone river meets the Mediterranean sea. This will set you back about $9, which is actually pretty reasonable as long as you understand that you're purchasing a flavorful and high-end garnish.
Fleur de sel is very delicate and will quickly dissolve, so it really should be added to a dish immediately before serving. One interesting use of fleur de sel is sprinkling it on candies, for example caramels, or other sweet items, like creme brulee, which both heightens and contrasts the sweetness.
It's best to store fleur de sel in a glass or porcelain container (i.e. a jar) with a lid.