Tequila
is Mezcal, based on my reading, but a very specific mezcal. Tequila is a mezcal made only from the blue agave plant in the region around Tequila, Jalisco, but spirits labeled simply "Mezcal" are often made using other agave plants and are from any region.
Tequila is prohibited by law from having "the worm". Only mezcal can have it.
Source:
http://en.wikipedia.org/wiki/Mezcal#The_worm