Why can the hot pepper burn?

What is hot in peppers is capsaicin, a chemical that elicits a sensation of pain when it bind the vanilloid receptors in the nerve endings (usually inside the mouth) of the trigeminal nerve. As it happens, all mammals have capsaicin receptors, but it was found, relatively recently, that birds do not. I have found that the seeds make the peppers even hotter is you cook with them.

It's caused by an irritating substance found in the skin of the pepper. I'm not sure of what kind of substance is that though