A small excerpt from this article:
Trying to identify the various types of powdered chile can be as confusing as U.S. foreign policy. In fact, I once precipitated a small-scale family crisis in a Mexican food store when I tried to purchase some pasilla chile. The 10-year-old clerk pointed to one barrel, his 11-year-old brother to a second, and their father — the store’s owner — to a third. The funny thing is, each could have been correct. There simply is no universally agreed upon nomenclature for chiles.
The wise shopper, therefore, chooses powdered chile by taste and smell. Place your finger in the spice and sample it, then sniff. If your sinuses clear, your vision becomes fuzzy, and your scalp begins to sweat, that particular variety may be more than you can handle! As a rule, gringos who are unfamiliar with very hot foods should avoid serrano and jalapeño chiles. I enjoy (and recommend) the fullbodied pasilla, New Mexico, and California chiles . . . but let discretion be your guide!