Mexican cooking runs on chiles — fresh and dried varieties each bring their own heat, fruitiness, or smokiness, and most recipes call for a specific one rather than "chili powder" in general.
Fresh: jalapeño (mild-medium, the everyday chile), serrano (hotter and brighter, common in salsa and ceviche), poblano (mild, thick-walled, used for chiles rellenos and turns into ancho when dried), and habanero (very hot, fruity, common in the Yucatán and in hot sauces).
Dried: guajillo (mild, tangy, the backbone of birria and pozole sauces), ancho (dried poblano — mild, sweet, raisin-like, key to mole), pasilla (dark, rich, mild-medium), chile de árbol (small, hot, used for spicy table salsas), and chipotle (smoked, dried jalapeño — sold plain or "in adobo" sauce).
Heat is measured in Scoville Heat Units (SHU): jalapeño runs roughly 2,500–8,000 SHU, serrano 10,000–25,000, habanero 100,000–350,000. If a recipe is too spicy, remove the seeds and white pith (venas) before cooking — that's where most of the capsaicin lives.
Buying tip: dried chiles should be pliable, not brittle or dusty — brittle chiles are stale and will taste bitter once toasted. Most Baja grocery stores and mercados sell dried chiles loose by weight.
Enjoy the flavors of Baja