As a partial answer to what makes Mexican food authentic here's a guest post by Susan T. about how Mexican food has been Americanized here in the States:
Think Taco Bell qualifies as Mexican food? Think again. While the menu may have some traditional Mexican favorites, they've been changed quite a bit over the years to appeal to the American palate. This is true of nearly every Mexican restaurant in the country. Fast food joints and Americanized Mexican restaurants are the only exposure many Americans have had to Mexican cuisine. Even those who don't speak Spanish know what tacos, enchiladas, and chimichangas are. But Mexican food didn't start out that way. Here are a few ways it's changed, and why.
Mexican food uses three basic staples—corn, beans, and rice. While it's sometimes eaten fresh, corn is most often used to make other things such as tortillas and tamales. Beans are cooked many different ways, and accompany nearly every meal. After corn, rice is the second-most-used grain, and is also a frequent side dish. These three items were the most abundantly available in Mexico, and formed the basis of the cuisine prior to the 16th century when Spanish settlers landed.
Spaniards brought many changes to Mexico, and the cuisine was not exempt. At that time, pastries with either sweet or savory fillings called empanadas were—and still are—very popular in Spain. The settlers also introduced chicken to the Mexican inhabitants, and soon a combination of traditional Mexican ingredients, chicken, cheese, and a tortilla was formed. It became the quesadilla, which still exists today, and can be found across the United States filled with all kind of ingredients.
Perhaps the single most important influence on Mexican cuisine has been the country's proximity to Texas. This is how the term Tex-Mex originated, and is used by many purists to describe any and all "Mexican" food in the United States to differentiate it from true Mexican fare. As Texas grew and developed, it became known for its meat. Cowboys on cattle drives needed simple foods that could be cooked fireside while on the trail. A pot of beans cooked in tomato sauce, a traditional Mexican dish called chili, could be satisfying, but it wasn't enough for Texas natives. They added meat and spices, and the dish became chili con carne, or chili with meat, which remains one of the most popular dishes in Texas to this day.
Through the years, Americans have added their own touches to Mexican food, either to make it more interesting, more flavorful, or just more palatable as true Mexican food can sometimes be bland. One example of this is the use of cheese not just as an ingredient, but as a food topping. Go to any Mexican restaurant in the United States, and you'll find tacos, enchiladas, beans, just about everything covered with mounds of melted, cheesy goo. Even salads get a healthy dose of shredded cheese sprinkled on top. While Mexicans will add cheese to some dishes, it's not used nearly as much in authentic fare as it is in Americanized dishes.
Probably no food served in Mexican restaurants is as American as the fajita. In fact, it's usually difficult to find it described in print as being Mexican, and is instead labeled as Tex-Mex. The dish's name is derived from the Spanish word faja, which loosely translates to skirt or strip, and describes the cut of meat usually used—skirt steak. Fajitas are also often made with chicken or even shrimp. The meat is marinated, grilled, then cut into strips and served on tortillas with grilled vegetables and salsa, another American invention. Fajitas first began showing up on menus in the early 1970s, and grew so popular so fast that they are now a staple on every Tex-Mex restaurant menu in the country.
There's no reason why you shouldn't enjoy fajitas, chili con carne, or any of the many other Mexican foods that have been Americanized in the United States. After all, they wouldn't have become so popular if they weren't so delicious. But if you ever travel to Mexico and have the opportunity to try some traditional Mexican food, don't pass it up. You may find you like it even more than the Mexican food you're used to.