The perceived "heat" of the pepper results from both the amount and the specific type of capsaicin. High temperatures, drought conditions, and nearness to full ripening all contribute to high levels of capsaicin and a correspondingly intense burst of heat! Some chilis are also genetically built to produce more of the chemical.
The specific type of capsaicin in a pepper can also be traced back to genetic make-up. These kinds of capsaicin cause different perceptions of heat--from intense but brief, to a slow build over time.
Poblanos are generally considered to be on the mild end of the spectrum, while habaneros are sure to knock your socks off!
In cooking with chiles, remove the innards of the chile and use just the outer fruit--but save the seeds and inner white tissue! Keep tasting for heat, and then add in minced pieces of the white tissue or a few seeds as desired.
Remember, you can always add more chiles in, but you can't take them out!
What are you favorite kinds of chile peppers?
Related: In Praise of Red Pepper Flakes
(Image: Steaming Chili Pepper on Fork by Howard Sokol, $29.99 on AllPosters.com)