Capsaicin is the active ingredient in chili peppers. It is an irritant to mammals and causes eye and skin irritation. Insects and birds are unaffected by capsaicin as the do not possess the receptors that cause the irritation in mammals and humans. Chili peppers have been thought to have evolved to repel mammals and not birds because mammals chew the pepper seeds and thus do not allow for plant reproduction, whereas birds swallow the seeds whole which pass undamaged and fertilized through their digestive tracts.
In chili peppers, capsaicin occurs at the highest levels in and around the seeds of the peppers, also known as the placenta tissue.
There are many culinary and medical uses for capsaicin, including aiding in cancer treatments and curing many digestive ailments. Common uses in food include making salsas, hot sauces, beverages, and many other culturally diverse dishes from around the world.