The Scoville Scale

In 1912 Wilbur Lincoln Scoville developed a method of measuring the heat of a chilli.   This was officially called the Scoville Organoleptic Test and involved a panel of five volunteers tasting different chillies added as extract in precise amounts into a sugar and water solution.  The point at which the heat of the chilli was no longer detected by the taster determined the rating.  So, if a chilli was no longer detected after dilution 100 times its rating would be 100 Scoville Heat Units or SHU.  If it was diluted 1,000,000 times before it was no longer detected it would be rated 1,000,000 and so on.  However, as you can imagine different people have different tastebuds and the ratings were very subjective.
Nowadays the technique is considerably more refined and scientific but again remains a little imprecise as an individual variety of chilli can have different levels of heat depending on many factors such as weather, soil, genetic composition and many other growing conditions.  For this reason the heat of a chilli is usually presented as a range rather than a single number.
High Performance Liquid Chromatography, HPLC for short, separates capsaicin form the other liquid components in the chilli and calculates this concentration in parts per million.  This unit of measurement is the American Spice Trade Association pungency rating or ASTA.
However, most people still prefer to refer to the Scoville Heat Unit in the UK at least.  To roughly convert the ASTA rating to the SHU we simply multiply by 15.
Pure capsaicin has a generally accepted SHU of around 16,000,000.
So, if capsaicin is our benchmark, how do other well-known chilli peppers rank alongside this?
Well, as you might imagine, the simple sweet pepper has an SHU of 0 and jalapeño an SHU of between 2000-5000.  The Cayenne pepper comes in at 30,000-50,000 and the Scotch Bonnet at 100,000-300,000.
The quest to grow the hottest chilli in the world is a fast moving story.  In 1994 the Californian Red Savina Habanero hit 577,000 SHU and the well-known Naga Jolokia from India topped that with an SHU of 855,000.  However, in 2012 the Trinidad Moruga Scorpio hit an all-time high of 1,200,000!
http://newsfeed.time.com/2012/02/17/worlds-hottest-chili-pepper-identified/
The hottest alkaloid in the natural world is believed to be the sap of some Euphorbias in Morocco which are around 1000 times hotter than pure capsaicin with a Scoville rating of around 16,000,000,000!