In this region, a taco is generally a diminutive corn tortilla heated on an oiled grill and filled with one of many meats, shrimp, or batter-fried fish.
If you're eating at an informal taco stand, your server may ask "Preparadito?"; he or she is asking if you want cilantro and onions. Then add your own condiments: salsa mexicana (chopped raw onions, tomatoes, and green chilies), guacamole, and pickled jalapeño peppers. Some restaurants go further with chopped nopal cactus and other items.