Armagnac is one of the most expensive types of brandy in the world. It is also one of the most closely-guarded products in the world. It is similar in style to cognac but it is produce in a different region, and is much more popular in France. The vineyards that produce armagnac are located chiefly between the Gers and Lot-et-Garonne regions, as well as the Landes. Roughly 40% of the armagnac produced never leaves the region of Gascony, and 80% of the remaining quantity never leaves France. The concepts of supply and demand drive the prices for armagnac to obscene levels in other countries.
Technically, there are three types of armagnac:
Bas (low) Armagnac
Haut (high) Armagnac
and Tenarèze
There is no differnce in quality ("low" does not denote a poorer vintage, nor does "high" refer to superior flavor). The armagnac region itself is actually divided into three subsections. Haut Armagnac is produced in the areas surrounding Auch as well as north and east of Nérac; Bas Armagnac is produced in the region surrounding Eauze, and Tenarèze is produced in the region south of Nérac.
An interesting side-note is that D'Artagnan, one of the three musketeers from Alexandre Dumas' novel was based on Charles de Batz, a native of Gascony in the Armagnac region.