Chile (or chili), for most of the world, refers solely to the smaller, hotter fruits of various species of capsicum plants.
Of 20 to 30 chile species, only five have been selectively bred and cultivated, mostly from the Capsicum annuum species. (Unless specified, those listed here are C. annuum.) Within each species are many varieties and subvarieties.
In terms of heat, in general, most large peppers are mild, particularly the broad, round shapes (bell and tomato types), and most small peppers are hot, especially the thin, pointed peppers (cayenne, pequin).
That said, there are exceptions at both extremes, especially among hybrids. Habanero (C. chinense) varieties come in fat and thin shapes. So far, they are all small and extremely hot.
Chiles thrive on exceptions.
Ancho and poblano have varied meanings: Different varieties bear those names, as does the broad category. Poblano also is a generic term for fresh peppers of that type, while ancho refers to the dried form.
All green peppers are immature, says Janie Lamson at Cross Country Nurseries in Rosemont, N.J. (north of Trenton). Left to ripen, they will turn one or more different colors en route to maturity - mostly in the yellow-orange-red range, but also purple, brown, near black, and even white.
As a consumer, look for clear labels on chile products, descriptive in-store signage, or a chart with color photos and detailed descriptions of popular varieties to use as a guide.
In the end, it's your choice, and at best an educated guess as to how hot a pepper will be.
Anaheim (or New Mexico). Long, thin pods, mostly flat and smooth (3 to 9 inches long, 1 to 2 inches wide). Most are mild to medium hot, bracketed by a few sweet and hot strains. Anaheim is also generic for green chile.
Ancho / Poblano. Large, wide and flat, with medium-thick flesh; 3 to 7 inches long by 2 to 3 1/2 inches wide. Mild to medium heat. Used in Mexican cuisine, roasted, stuffed. Called poblano when fresh, ancho when dried.
Banana (or Long Wax or Hungarian Wax). Sweet to medium-hot when immature yellow, but a mature red may be very hot; 3 to 8 inches long, 3/4 inch to 2 inches wide. Often pickled. Use in raw salsas.
Cayenne. Long, thin, pendent (or tiny upright) pods; 1 to 10 inches long, 1/4 inch to 1 1/2 inches wide. Medium to hot with some very hot (small); some mild in every size, and a few sweet. Used in Mexican, Indian, Asian cuisines, stir-fries (remove before serving); dried or pickled.
Cubanelle. (Italian frying pepper) Long, large, light green (4 to 10 inches long, 1 to 2 1/2 inches wide. Greenish yellow/pale green to red; sweet-mild. (Thin Italian roaster hybrid is medium hot.) Use for roasting, stuffing and stir-fries.
Habanero. (C. chinense) Among the world's hottest chiles. Varied shapes, as in the tam-o'-shanter look of Scotch bonnets; 1 to 3 inches long, about as wide. Use in sauces or pickled.
Jalapeño. Pendent, thick- fleshed pods; 1 to 4 inches long, 1/2 inch to 2 inches wide. Mostly medium to hot, with a few sweet-mild hybrids. Used in fresh salsas, Mexican cuisine, canning. Called "chipotle" when smoked.
Pasilla. Long, thin, pendent pods; 5 to 9 inches by 3/4 inch to 1 1/2 inches. Mild to medium heat, flavorful; often confused with wide poblanos and anchos. Used in Mexican cuisine, sauces, dried and powdered.