One thing I noticed at the Markets in Mexico was the large display of chilis both fresh and dried. As you eat the different regional cuisines you realize that the dried chili is primarily the workhorse behind those complex flavors in Mexican cooking.
When you pay attention to most Mexican restaurants in America, you notice more attention toward the fresh chilis such as, serrano, jalapeno, poblano, and habanero. These are all wonderful and serve an important purpose in Mexican cuisine. However, understanding how to use the different dried chilis is where the rich, smokey, chocolaty, and spicy flavors come from.
One important note to keep on mind when working with dried chilis. Most of the time you will do a minimal of two steps when using the different varieties, 1- remove the stem and seeds, and 2-lightly toast the chili. When you are toasting the chilis be careful not to over toast or burn the chilis. You are not trying to give the chili any extra color but heat up the oils in the chilis, so the flavors come out more when you cook with them. Using a medium heat and a dry pan or Comal is the best way to achieve this.