Mezcal vs. Tequila

Both spirits are distilled from agave plants, but tequila can only be distilled from certain agave plants. In order for a spirit to be legally advertised as tequila, it must be made from the Weber blue agave, and grown in specific territories recognized by the General Declaration on the Protection of the Appellation of Origin Tequila, as put forth by Mexico’s Tequila Regulatory Council.

Anything else made from the agave plant – even to near-identical standards – is a mezcal, which is technically a blanket term for any spirit distilled from agave.

Agave hearts, or piñas used in the production of either tequila or mezcal can be cooked before fermentation, though those used for mezcal are more often roasted in underground pits, imparting the finished product with generally more of a smokier aroma and taste.
Following the fermentation and distillation processes, tequilas and mezcals can be aged to varying degrees before bottling, resulting in multiple distinctions. The distinctions include Blanco (bottled within two months), Reposado (aged between two and 12 months) and Añejo (aged longer than 12 months), with the younger tequilas generally being better for mixing, and the older better for sipping.
All tequila is mezcal, but not all mezcal is tequila.

Incidentally, Mezcal is Spanish and mescal is English spelling.