Mezcal is a distilled spirit made from the agave, a spiny succulent. The production of mezcal is centered around Oaxaca, and there are governmental regulation in place to protect the identity of mezcal production as separate from that of tequila. While both tequila and mezcal share NOM (Norma Oficial Mexicana) status, mezcal production is also subject to COMERCAM (Mezcal Regulatory Council) approval.
While people have been distilling the juices of the agave for centuries, mezcal production remained the province of small, loosely organized producers until recently. In the late 1990's, there was a shortage of agave for tequila production, and many tequila producers shipped agave from Oaxaca to make up for low supply.
Mezcal production differs from tequila production in a few, very important ways. First, mezcal production done on a smaller scale and less industrially than most tequila production. Second, the type of agave used is different - mezcal can come from up to 30 different varieties of agave, though most are espadin. Tobala, a wild agave, is sometimes used in mezcal production, but these bottlings are fairly expensive. Third, where the piñas (the tuber-like root of the agave) for tequila production are steam cooked then shredded, piñas for mezcal are roasted in a stone pit covered with earth so that the sugars caramelize, then are ground on a stone wheel. The ground piñas are fermented into a pulque-like liquid called tepache, which is then distilled, usually once (though some producers distil their product two or three times), and then finished for bottling.
The closest similarity between tequila and mezcal production is that of cognac to armagnac. Both armagnac and mezcal are widely seen as rustic, country cousins of the more suave, polished cognac and tequila, but both have their admirers.
Because of its intense smokyness, mezcal's flavor profile is somewhat like Islay Scotch with the fruit and salt undertones of tequila.
Labelling of mezcal is similar to that of tequila, though there are some important differences. There are two main types of mezcal: 100% de agave, and Mixto, which mandates a minimum of 80% agave, instead of the 51% required of tequila. Unaged mezcal is called abacado, which can have coloring and flavorings added to it. Colored abacado mezcal is sometimes called dorado. Aging the mezcal in barrel for between two and nine months will make it reposado, or madurado. Over one year in a small barrel will make it añejo mezcal.
Mezcal also has some terminology that it does not share with tequila. Crema de Mezcal is a liqueur, most often fruit or nut flavored, and around 40 proof. Tobala is made from only the wild tobala variety, and is usually unaged. And Con Gusano denotes a mezcal with a worm in the bottle, though the worm imparts no flavor and is simply a marketing gimmick.
Monte Alban is a solid, entry level mezcal that is widely available. In a more premium tier, Los Danzantes, Scorpion, and Los Nahuales are good brands. In an ultrapremium tier, Del Maguey and Ilegal add traditional processes, varietal and village specificity and artisinal quality.