Basically, in case you don't know, tortillas are round and flat and very thin. They need to be heated up on the comal, a kind of flat metal plate used on top of the stove, and eaten warm and soft. They are served in a special kind of basket wrapped in a cloth to keep them warm and accompany all different types of dishes instead of bread. You can even make tacos by putting a small amount of whatever dish you're eating - roast chicken, steak, scrambled egg, etc - on the tortilla and folding it in half or rolling it up. Don't do this in "polite company", though, as it's considered bad table manners!
You can buy tortillas prepackaged from any shop, or you can buy them by the kilo from a tortillería, a little shop containing a noisy machine that churns out corn tortillas onto a conveyor belt. The best tortillas, however, are handmade; slightly thicker and with a much better flavour and texture.
It seems it was the Aztecs who "invented" the tortilla over three millennia ago and who also developed the process of nixtamalizacion which is still used today, although generally on a more industrial scale. This word originates from Nahuatl (the Aztec language); nixtli, which means ash or lime, and tamalli, which means corn dough. This ancient process basically consists of cooking the corn in water with lime (calcium hydroxide, not the fruit!) and has several functions, including increasing the nutritional value of the tortilla and making the proteins more easily digestible, prolonging shelf life and improving the flavour.
Apart from being an accompaniment to the main dish, a lot of classic Mexican dishes and snacks are made with tortillas. Quesadillas are the easiest thing in the world to make and are so delicious: just put some meltable cheese on one half of a tortilla, fold over the other half and heat on the comal (or fry) until the cheese melts. I like my quesadillas toasted till they're crispy!
Tostadas use a crispy toasted or fried tortilla as a base on which to pile on different toppings.
Sincronizadas are another simple snack to make, usually with flour tortillas. Put cheese, ham, avocado, tomato and lettuce between two tortillas and heat on the comal.
One of my favourite soups is Sopa de Tortilla or Sopa Azteca. It's made with strips of fried tortilla in a tomato and chile pasilla broth (unfortunately I don't have a recipe as I've never made it myself), served with chunks of white cheese, avocado and cream.
These same fried tortilla strips or triangles are used in a typical Mexican breakfast - chilaquiles. In this dish they're covered in a green or red salsa and you can add shredded chicken or scrambled egg, cheese, cream and avocado. It's one of the traditional hangover cures - the hotter the salsa, the better!
And of course you have the ubiquitous tacos; small tortillas usually with a meat filling, although you can get other types of fillings. You have to add salsa and lime juice to your tacos, and maybe some chopped onion and coriander, too. You can buy them from a taqueria or a taco stand in the street; they're everywhere!
So, there you have it - cheap, versatile and nutritious. Provecho!
I got some of the facts and figures mentioned here from this article (in Spanish) about tortillas de maiz.
You can read the first post in this series here and my post 11 years in Mexico has more about my food adventures from my first few months in Mexico.