Guajillo chilies, are long, dried, red hot mirasol chili peppers (part of the highly variable species Capsicum annuum). Guajillos contribute a sharp, fruity, somewhat tangy flavour to dishes without contributing a great deal of heat. They’re a fairly common and popular pepper in Mexican cooking, though they tend not to be as quite well-known/used outside of Mexico. Guajillos are moderately spicy, and fairly comparable to a cascabel or chipotle pepper of the same size.
Guajillos are generally available whole (as shown in the picture), and ground or crushed, though it is difficult to find crushed guajillos that have specifically been labeled as such.
Guajillos are also called guaque chilies in Guatemala.
Scoville Heat Units