All tequilas are mezcals, but not all mezcals are tequilas. 1st, the region: Tequila can only come from specific places (Michoacán, Guanajuato, Nayarit, Tamaulipas and Jalisco, where the actual town of Tequila is), when Mezcal can comes from anywhere else in the Country (yet, most of the production comes from the Southern Oaxaca region).
Then, there is the varieties of Agave used: Tequila can only be made with blue agave (agave tequilana), when Mezcal can be made from more than 30 varieties of agave.
Finally, the method: Tequila is typically produced by steaming the agave inside industrial ovens before being distilled two or three times in copper pots. Mezcal, on the other hand, is cooked inside earthen pits that are lined with lava rocks and filled with wood and charcoal before being distilled in clay pots. While some large-scale mezcal producers have adopted modern methods, artisanal mezcal makers continue to use this more traditional method, which is the source of the smokiness commonly associated with mezcal.
David Stephan – Resident Wine Geek