Most peppers are a variety of a single species (Capsicum annuum), but like dogs, chile peppers exhibit amazing variety within the species. That's great for Chowhounds, who enjoy experiencing the subtle differences in flavor and heat among the many varieties. Peppers develop local variation very easily, says paulj, and they then hybridize to form even more variety. But that variability can lead to confusion. "In traditional parts of Mexico (e.g. Oaxaca, Chiapas), local chiles could vary from village to village, along with names and linguistic dialects," says paulj.
Dried chiles for use in Mexican cuisine can be particularly hard to tell apart, especially for people just learning the cuisine. There are two dried chiles that look particularly alike: the ancho (the dried version of the poblano pepper) and the pasilla (the dried version of the chilaca pepper). To add to the confusion, both dried peppers are frequently mislabeled, with the names used interchangeably by producers, says JuniorBalloon.
The best way to tell the difference, regardless of the label, is that the ancho pepper has a subtle reddish tinge to it, and the pasilla is more brown to black, says neoredpill. They are both mild, but the ancho has a sweeter flavor distinct from that of the pasilla. "Ancho chiles are certainly more common and widely used in this country, but pasilla is used for authentic mole sauce," says neoredpill. "Truthfully, most people would never know the difference, which is why marketers get away with the mislabeling of ancho chiles as pasilla."
qbnboy90 agrees with the color and flavor characterization. The color difference is barely noticeable unless you're looking for it, he says, but the black/red difference becomes especially pronounced when the chiles are cooked in a sauce. For those new to Mexican cooking, try making two dishes: "one with just ancho, and another with just pasilla using the same recipe as a base and compare, you'll see the difference," qbnboy90 recommends.