Mezcal, like tequila, is a spirit made from agave, a big spiky desert plant which, contrary to popular understanding, is not a cactus, but is closely related to a lily. It's definitely not mescaline, so you can save your Fear & Loathing quotes for a glass of Wild Turkey.
While tequila is required to be made from blue agave, most (but not all) mezcal is made from a variety called espadin, and it must be made in a certain region in Mexico, the most well-known of which is Oaxaca.
Espadin takes eight years to grow to maturity, though other varietals can take longer. Once it’s mature, the spines are cut away, exposing the hearts -- piñas -- which are buried in smoky fire pits and covered. The agave cooks from anywhere between three days and two weeks, as the starchy plants are converted into soft, sweet, pulpy fruit. (Tequila, on the other hand, is cooked for far less time in a more conventional oven.)