Capsaicin

Capsaicin is the compound in hot peppers responsible for the sensation of spiciness. The molecule contains a long, hydrophobic tail which makes it difficult to remove with water. 

The spicy sensation is caused by capsaicin interacting with nociceptors, receptors that govern the perception of irritation, pain, and temperature. The capsaicin triggers a perception of heat and pain, leading to what we call "spiciness". Compounds which do not have a specific taste but are rather associated with a sensation are called chemisthetics.

Spiciness is measured with the Scoville scale, which is the dilution ratio required for the spice to no longer be detectable. A habanero pepper has a Scoville rating of about 300,000, meaning you would need to dilute 1 gram of habanero in 300,000 grams (3000 liters) of water to no longer detect the spice. Pure capsaicin is rated 16,000,000 on the Scoville scale. A good way of dealing with the spice is rinsing your mouth with whole milk.