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.