What is a Taco, Anyway?
When asked to explain what a taco is, most Mexican taco experts start out by pointing out what a taco is NOT: hard and crunchy “taco-shells” holding ground beef, ice-berg lettuce shavings with diced tomatoes, and cheddar cheese. The kind of things you can get at a famous fast-food chain whose name sounds like a phone company. Now this may stir up some controversies, as many people like to make tacos at home, following this example. And now I'm telling them that those things are not really tacos??? For the sake of peace, let me just say that these “American Tacos” can be quite delicious too (the home-made ones, not the fast-food), even if the idea is not the same in Mexico.
Instead, a taco is understood to be basically any food (usually meat, but not necessarily) that you pick up and eat with a tortilla. So in fact, a taco is a very broad term. It could be as simple as a tortilla with some re-fried beans smeared on it, and a bit of salsa to add some taste. More commonly, however, tacos hold a type of meat. Typically the tortillas used for a taco are small (about 10 cm or 4 inches in diameter) and to make sure they won't fall apart, two are used together for one taco. At most taquerias the basic options include suadero (ground beef), longaniza (same as chorizo, except for the sausage is not tied off into separate sections), campechanos (a mix of the two) and the most famous al pastor.
Lebanese Shwarma disguised as a Mexican
Visitors to Mexico may be surprised to see so many “kebap spits” turning, not only at small food stands on street corners, but even in high-end restaurants. The famous tacos al pastor are a legacy of Lebanese immigrants who came to Mexico around the beginning of the 20th century. By now, their version of kebap has found its way into the heart of Mexican cuisine, where it's been lovingly kept as a cultural icon. Marinated strips of chicken, lamb, or beef are stacked on a vertical spit, heated slowly by a flame on one side. An onion and a pineapple are often on the spit as well, to let their juices infuse the meat. As it slowly gets cooked, pieces are sliced off from the outer surface and placed right on the tortillas, sometimes with a piece of pineapple, if the customer so desires. 
Other Taco Options:
Many taco stands offer many additional choices for their tacos: tripa (tripe) is very popular, and also one of my favorites. Other innards include trompa (pork nose), lengua (beef tongue), ojos (beef eyes), sesos (brains – not sure, but I think it's beef), among others. Apart from these options, there are specialized tacos and regional delicacies, such as barbacoa, carnitas, or cochinita pibil. Since in all of these cases it is not the taco, but what's inside that is important, I want to address these dishes in separate articles.
Certain places also sell tacos de guisado, which are cooked dishes, served once again on two tortillas. These could be as basic as some fried rice with an egg, or be a combination of any meat and/or veggies. These type of tacos are the perfect opportunity to sample a variety of dishes in one sitting (or rather standing), as each taco has about a spoon-full of stuff on it, and it generally takes about 2-3 bites to eat.
Finally, there are also tacos de canasta, or basket tacos. These tacos have been prepared at home, and sold on street corners or metro stations straight from the car trunk or a bicycle rack. The “baskets” used nowadays are in fact styrofoam boxes to keep the food warm. Often times these vendors can pop out of nowhere, and disappear just as quickly, as many of them operate without permit. Their tacos, however, are just as delicious as they are low-cost, and just as with most stalls of street-meat, I have never had any bad experiences with them in my five years of playing Mexican Roulette.
Don't Forget the Condiments
One of the nice things about eating tacos are the condiments. Sure, the tacos could be tasty enough without them. Still, it's a good idea to make use of at least some of the condiments provided, as they not only enhance the taste, but additionally help prevent Moctezuma's Revenge. The verduras (chopped onion and cilantro mix) and the limones (limes to squeeze the juice onto the taco) are certainly useful for this. Then there are the salsas. The red one is made with red peppers and tomatoes, the green one with green peppers and green tomatoes. Since there is no general way of knowing which is hotter, it's a good idea to ask the taquero: ¿Cual pica más? Sometimes there may also be guacamole among the condiments, but be careful: many times it can be hotter than the green salsa, or it just may BE the green salsa. So it is wise to try just a little first. Another frequent condiment is the pico de gallo, which is chopped tomatoes, chili peppers, onions, and cilantro. In fact, it is the same thing as salsa, only that it has not been through the blender.
A Word on Food Quality
I can imagine that my descriptions of crummy looking street stalls, tacos sold from a box on a bike, and my mention of Mexican Roulette, may have caused one or the other reader to get a bit suspicious regarding the quality of the food. After all, the stereotype of an upset stomach or even serious food poisoning is all too common, and where would we be without regulations? Well, to add my own experiences, I have to admit, I'm no stranger to Moctezuma's Revenge. In these five years that I've been staying in Mexico I have come down with some horrible stomach issues. However, each of these four cases was after eating at a some ritzy looking joint, where the white-shirted waiters bend over backwards, and call me señor instead of amigo. As a result, I generally prefer eating on the street, where I can't recall any such experience, ever.
Now is that really surprising? Let's consider the difference: An expensive restaurant is much better prepared to deal with such an incidence than a taco-stand on the corner. A corporate franchise even more so. The legal department and the PR crew can iron out any image issues and a few sick people won't affect the share prices much. For your local taco-stand, however, it is a definite death sentence if word gets out on the block that someone might have gotten sick. So it is in their own best interests to keep the food clean and tasty, which is not even that hard to enforce, given the small size of the operation.
Now that you know the basics, if you are coming Mexico, I can only recommend you to Eat the Street Meat, and try as many types as you can.