So what exactly is the Scoville Heat Scale?
The "heat" of chili peppers is measured in Scoville Heat Units (SHU). Bell peppers rank at 0 (SHU), jalapeños at 3,000–6,000 SHU, and habaneros at 250,000 to 570,000 SHU.
The record for the hottest chili pepper was assigned by the Guinness Book of Records to the Naga Jolokia, measuring 857,000 SHU. Pure capsaican, which is a hydrophobic, colorless, crystalline solid at room temperature, measures 16 Million SHU!
Putting it simply, the Scoville Heat Scale measures the number of drops of water you would need to drink to take away the heat of one drop of the sauce. So if a sauce is rated at 1 Million Scoville Units; you would need to drink 1 Million drops of water to take all the heat away!