In todays definition, Mezcal is a spirit made from distilled agave juice produced only in 9 states within Mexico. In the past any spirit distilled from an agave was called mezcal. We like to say, tequila is a type of mezcal.
Mezcal are still produced in an old fashioned way. Most brands still roast their agave in an earthen pit which imparts a smoky characteristic, wild fermented and distilled in small copper or sometimes clay pot (aka Filipino still).
Mezcal are not limited to one type of agave, which means it can be distilled from a variety of different agave. Just like a wine different grapes taste different, different agave distilled spirit will taste very different.