The Scoville scale measures the hotness of a pepper.

A "Scoville Unit" is actually a measure of capsaicin, the chemical that makes a hot pepper hot. Most capsaicin is found in the ribs and seeds of a pepper, which is why seeding a pepper makes it's heat less potent.

You might have noticed a Scoville rating on your bottle of hot sauce. Original Tabasco has a rating of 2,500 to 5,000 Scoville units. The hottest readily available peppers, Scotch Bonnet and habaneros, share a rating of 100,000–350,000. India's Bhut Jolokia pepper is in the Guinness Book of World Records as the hottest known pepper. It measures 1,000,000 heat units. (Pepper spray weapons hit 5,300,000 units.)

The rating of a bell pepper? Zero -- no heat from this pepper.

Reference: Habanero vs. serrano vs. jalapeño
Reference: How to dice a jalapeño
Reference: Help! I ate a hot pepper!

(image courtey of wikipedia)