Mezcal is very similar to Tequila. Tequila uses the starches and sugars of the Blue Agave plant to produce a spirit. However, Mezcal uses any and all types of agave. It can also be made anywhere within Mexico, where Tequila can only be made in a few small specific regions.
Mezcal is produced by taking the agave plant's 'heart' (the fleshy/starchy centre of the plant) and roasting it in either a fire pit or clay oven. These cooked 'hearts' are then crushed into a thick powder/paste which is then fermented with wild yeasts.
The fermented agave mix is what is then distilled. This will happen in a Pot Still which allows for a more textural and aromatic spirit. More traditional producers will just have the single distillation but many more modern, or refined, producers will undergo at least one re-distillation process.
Generally speaking Mezcal isn't aged in oak (unlike the majority of top end Tequila). Contrary to popular belief that Mezcal is Tequila's lower quality brother and it always has worms in it. This simply isn't true! Mezcal, when done well, produces a highly refined spirit.
NOTE: It is only a particular type of Mezcal: 'Oaxaca', which has the infamous worms in the bottle.