This is the most typical and a little exotic staple you will find in most of the places you visit in Mexico. The origin is not Aztec, though.
In any little town you will find a taqueria offering Tacos al Pastor, you will notice immediately as they put the vertical rotisserie at the entrance of the restaurant to attract the clientele.
Traditionally the rotisserie would be layered with charcoal brickets, and that heat is responsible for slow-cooking the meat. Nowadays the use of gas and refractory bricks is necessary due to the volume of tacos needed. The meat is usually marinated pork, with the achiote paste one of the main ingredients, which gives the meat an orange-redish look, but it depends solely on the cook preparing the meat. The meat rotates slowly and as each section is cooked the "taquero" will slice a thin portion out of the "trompo" (stack of meat) and put it on the tortilla.
A little bit back in time for the origins of this Mexican specialty. In the end of the 19th century a family from Lebanon came to Mexico and brought their Kebab style tradition, here after many many years and cooks the recipe was transformed from lamb meat to pork, and adding the achiote marinade and topping the transformation with a chunk of fresh pineapple at the top of the rotisserie to add a touch of tropical flavor, another Mexicanization of the long almost-forgotten recipe (if you want the original one, try the Tacos Arabes, no marinade no pineapple).
Pineapple? You would ask, well, yes, its flavor combines very well and its sweetness will balance the fieriness of the hot salsa you will have your taco smeared with. Some skilled taqueros will flip the pineapple on the tortilla with a flip of the wrist making it a show like the Japanese Teriyaki chefs.
One special note: in Mexico we put Limón (lime for you) in almost all of our food, its acid flavor will merge so nicely with the meat that you will wonder why you don´t have this at home daily!!!
Another special note: Salsas. This is a topic not to be missed, in Mexico salsas are usually hot and very spicy no matter what the guy in the next table tells you, or if the cook says it´s mild, NEVER believe them. First you should put A drop in your food to measure the hotness of the salsa. The color has nothing to do, nor the size of the chile peppers, there are some red ones so small that they look like toy chiles, but are really powerful. In a good taqueria you will have at least two different kinds of salsas: roja (red) y verde (green), the color depends on which chiles were used. If you only find one kind of salsa it means that you arrived very late and the favorite of the clientele is gone or you got a so-so taqueria. Some places carry the salsa verde con aguacate (avocado) but don´t think that it will be safe because you like guacamole, it can be as fierce as the other salsas.
Remember, always try a drop of each salsa before putting a spoon full of it in your taco. Buen provecho.