Tequila: Tequila is a distilled beverage made from the blue agave plant, primarily in the area surrounding the city of Tequila 65 km northwest of Guadalajara, and in the Jaliscan Highlands (Los Altos de Jalisco) of the central western Mexican state of Jalisco.
There are four main categories of Tequila: Blanco - These white tequilas don't spend any time oak aging after distillation and present the purest agave character. Reposado - These "rested" tequilas spend at least 2 months in oak barrels. This imparts a golden colour and a more complex oak character. Añejo - As the name suggests Añejo (Spanish for year) tequila spends at least 12 months in oak barrels. These present the most oak character of the tequilas. Extra Añejo - These will spend at least 3 years in barrel before bottling.
The aged tequilas can also be charcoal filtered to remove the colour, in which case they would be called cristalino.
Mezcal: Agave spirits made outside the regions legally able to market their products as tequila are called Mezcal. Traditionally Mezcal has been thought of as a substandard quality spirit, and while that certainly can be the case, there are plenty of examples of excellent Mezcal on the market around the world. There can be more variety in the flavours and character of Mezcal due to the wider geographic area in which they are produced.