It is very unfortunate that Mezcal suffers from the reputation it has. Up to now most of the Mezcal in the United States was low quality spirit meant to be drunk as quickly as possible to get to the worm at the bottom. It was almost exclusively vile stuff that only frat boys drank on dares. While tequila suffered the same problems and years ago moved on to a much improved status in both quality and brand choices, Mezcal is lagging far behind in image and respect. Which is a real pity given what good Mezcal is like. The following is an attempt to address some of the finer points.
Mezcal is a distilled spirit from agave. Tequila is a form of mezcal but can use only the blue agave (Tequilana Weber) where Mezcal can use 18 different varieties. The most common are the Espadin (Angustifolia Haw) and Tobala (Potatorum Zucc).
The Pina (or Pineapple as it is called) is baked underground in a pit for days to caramelize the starch into sugars then the cooked pina is crushed either/both by stone wheel or by pressing it with bare feet in a vat. The mashed pinas and water are then placed in clay or bronze tanks covered by dirt and heated by logs to distill.
To compare and contrast Mezcal from Tequila; Mezcal is produced in small lots and bottled at point of origin. The process is very labor intensive and cannot be scaled up. Tequila is produced in factories for the most part, in a very different process and usually shipped in bulk to be bottled elsewhere.
The three types or age classifications:
 Joven: Fresh from the still also known as Blanco or Silver, 
Reposado: Rested in Oak barrels(<200Lt each) for min 2 months, 
Anejo: Rested in Oak barrels(<200Lt each) 12 months or more
, Anejos older than one year usually have a age statement on the label as to the actual age.
 Unlike Tequila, Mezcal usually has a very smoky flavor because of the way it is prepared, it is in some ways like comparing a smoky single malt scotch whisky to a bourbon. Similar in some ways, quite different in others but both can be quite good.
 Now with the introduction of quality handmade mescal we hope that it will gain the respect it deserves.