Capsaicin is the most common chemical compound associated to the heat from a chile. Capsaicin belongs to a family of chemicals called capsaicinoids which are produces by chiles. Capsaicinoids bind to nerve receptors on the tongue or back of the throat, and they allow the flow of calcium into the cell causing a pain signal to be transmitted (or so I'm told).
In 1912, Wilbur Scoville developed a subjective method of ranking chiles. Scoville mixed ground chile in a simple syrup (sugar and water solution) and had a panel of tasters taste the solutions. The ratio of simple syrup to chile where the tasters were unable to taste the chile spiciness was the rating given to the chile. For example, a serrano chile might need 8000 parts simple syrup to 1 part chile before you would be unable to taste the chile.
As you can imagine, this system seems a bit too subjective. Everyone has different tolerances for tasting spicy foods, the panelists may have grown up eating chiles and become desensitized, and the process of tasting chiles probably helps desensitize the taste buds. The solution? High-Performance Liquid Chromatography.
High-Performance Liquid Chromatography (HPLC)
Becoming popular 1970's, HPLC is a method of separating compounds within a solution under high pressure. Once the solution has been separated, the parts can be identified and quantified. Applying HPLC can be used on ground peppers, chemists can determine the capsaicinoid concentration in parts per million. The capsaicin concentrating in parts per million is directly proportional to the Scoville rating system - by a factor of approximately 16. Thus, a capsaicinoid level of 200 parts per million results in a Scoville rating of 3200. Pure capsaicin would then have a Scoville rating of 16 million. The downside of HPLC is that it is quite expensive compared to having a bunch of people tasting chile flavored Frutopia...
One of the problems with scoville ratings is that no two sources seem to agree just how hot a chile is. Here's some of the examples that I've managed to collect to provide an idea of how the varieties stack up against each other.