Mezcal is actually the category that encompasses tequila. All mezcals are made from agave plants — tequila is made from one particular species, the blue agave, while smokier mezcals are made from one of the other 150 agave species native to Mexico.
Mezcal gets its smoky flavor during the production process when the hearts of the agave plants are cooked in pits in the ground. The cooked agave is then crushed, combined with water and allowed to ferment. A good mezcal will tell you the names of the agaves used to make the spirits and the bottle should say whether it was distilled in a clay or copper pot. Clay pots produce earthier mezcals, while copper makes for a smoother spirit.