Tequila is a spirit made from the Blue Agave plant.
Tequila in some form has been made for centuries. The Aztecs as far back as the 1st century AD produced a milky brew from the local agave plant. The Spanish, who landed in Mexico in the 16th century, discovered a type being made by the local Ticuilas Indians, from whom the name derives.
What is tequila?
Today, genuine tequila is made only from a species of agave called the Blue Agave plant, and chiefly in only one state in Mexico: Jalisco. Mezcal, which is similar, may be made from any of several agave plants.
The plant is matured for a decade or more, then stripped of its flower and grown fat, then diced. The pieces are baked in steam ovens to convert plant starches into sugars, which are part of the extracted juice. Mezcal, by contrast, is baked in charcoal ovens, giving it a more full-bodied flavor. The liquid is fermented, then distilled.
The final result is generally around 55% ABV (alcohol by volume). The other key figure to watch for is the percent of agave. The finest tequilas are fermented 100% agave juice. Others may be diluted by as much as half. The latter is called a mixto, which may or may not be noted on the bottle. But 100% pure will be labeled as such.
The spirit is a pure, clear liquid. Colored tequilas get their amber or yellowish cast mostly from added caramel. That additive isn't necessarily a sign of poorer quality. To each his own. A small amount of the tint may be the result of aging in oak casks, which - like whiskey and others - lends overtones to the drink.
But unlike whiskey, scotch or bourbon, long-term aging isn't generally done, nor is it a mark of excellence. Aging tequilas more than a few years causes the vegetal flavors to get overwhelmed. Many high quality tequilas actually see much less, a few months at most, some going straight into the bottle. Silver or Blanco tequilas will usually age no more than 60 days, and that in stainless steel barrels. But they're often intended to be used in mixed drinks only.
The casks used, not surprisingly, have an influence on the flavor of the final output. Many use once-used Bourbon casks, which are typically American Oak, sometimes charred on the inside. Others may use casks used to age sherry, which produces a slightly different result.
The careful observer can readily make out all these notes in a fine tequila, since there's little else to mask the aromas. Some tequilas, though, have been pre-blended with fruits, sherry or even prune concentrate, making it harder to detect some of the subtleties. Again, to each his own.
There's also no crime in using even the finest tequila to make a cocktail. After all, the point is to enjoy and how better to make a great mixed drink than starting with the finest ingredients? It's still the real thing.