Capsicum Annuum. Pimiento de Padron chili peppers (aka pimento de padron or just "padron" peppers) originate from the province, Galicia, in the northwestern Spanish municipality, Padrón. They are small to medium-sized peppers averaging about 3 inches in length. The color ranges, starting out bright green and maturing to a vibrant red. The interesting thing about padron peppers is that most of them are very mild peppers with no heat, but a small percentage of them will give you a shock of heat. The only way to know if a padron is hot is to give it a taste. It all has to do with the peppers' particular growing conditions.
Pimiento de Padron chili peppers are traditionally prepared as tapas, with a quick blistering or frying of the peppers, then tossing them with salt. I had them this way in Spain and loved them. However, they are great for incorporating into other dishes in other ways. I grow them sometimes in my own garden and prefer to let them mature to red for a bit more sweetness. However, a purist would say that if the peppers are not grown in Padron, Spain, they are not true padrons. They still taste great to me.