The Scoville scale, named after American pharmacist Wilbur Scoville, is a means to measure the relative "hotness" of chili peppers. 

The units of measure are the Scoville Heat Unit (SHU) which indicates how much capsaicin (the compound responsible for "hotness") found in peppers.  While this scale is not as precise as scientific measurement methods like high-performance liquid chromatography, and kind of subjective, it provides a reasonable comparison.

The following chart gives the Scoville ratings for various common peppers.

Scoville Heat Units (SHU) Pepper
15,000,000 - 16,000,000 Pure capsaicin.  Don't put this on your food.
1,500,000 - 2,500,000 California Reaper. This is a cross-breed between the bhut jolokia (ghost pepper) and a red habanero. It was rated as the world's hottest pepper by the Guiness Book of World Records as of August 7 2013.
855,0000 - 1,000,0000 bhut jolokia pepper (also known as Naja jolokia or the ghost pepper)
100,000 - 350,000 Habanero Pepper
100,000 - 325,000 Scotch Bonnet Pepper
30,000 - 40,000 Cayenne Pepper
10,000 - 23,000 Serrano Pepper
2,500-8,000 Jalapeno Pepper
1,000-1,500  Poblano Pepper
100-500  Pimento pepper
0 Bell pepper