Popular Mexican food is quite unlike the traditional Mexican cuisine and can be a far cry from tasty and brightly decorated authentic Mexican food. What is meant now under the word-combination "popular Mexican food" is Tex-Mex and food served in numerous Mexican style restaurants and fast-foods.
The dish that is believed to be Mexican and is very popular is taco. Many people expect it to be a hard, big U-shaped thing staffed with beef, lettuce, sour-cream and yellow cheese, because this is exactly the way tacos are served in any European and American Mexican food restaurant. But not a single Mexican will recognize a taco in it, because Mexican tacos are at least five times smaller, made of soft corn-baked tortillas filled with some pork, seafood or chicken. And of course there are a lot of very peculiar kinds of tacos in a genuinely Mexican menu: tacos made of cheeks, eyes and udders (carnitas) or with pure pork loin (macisa). But Tex-Mex or popular Mexican food is ignorant of such brave peculiarities.
Tortilla is sometimes mistaken for an omelet, as it is in Spanish cuisine, but actually tortilla is flattened Mexican bread, made of corn (approximately size of a CD) and it is used for tacos and some other dishes. Quesadilla is a pretty substantial dish. It is a rolled tortilla stuffed with cheese and meat and fried hard. Longaniza de Valladolid belongs to popular Mexican food too. It is a sausage, smoked and formed into skinny and short portions.
And of course it would not be an article about the Mexican food without mentioning the condiments. Chiles, salsas and herbs are capable to make even the plainest dish delicious. And in fact no dish in Mexican menu is eaten without a condiment: it can be a drop of lime or some extra salsa or a dash of chiles. That is why on the table of a good Mexican restaurant you will always find several little bowls with coriander, lime juice, salsa mexicana, spring onions, salsa ingles and of course guacamole.