In 1912 Mr. Wilbur Scoville developed a method for determining the spicy heat of chile peppers by measuring the amount of capsaicin present. He came up with Scoville heat units (SHU), which are an indication of the relative spiciness of various peppers. Mr. Scoville's method relied on human tasters, so some subjectivity was involved. He would dilute an oil extract from a dried pepper and add it to a sugar / water mixture.   He would keep adding more oil extract until the tasters registered the heat.

For sweet peppers, such as bell peppers, there is no capsaicin present so the Scoville rating is zero. For really hot peppers such as the habañero, the rating is 200,000, meaning the oil extract would have to be diluted over 200,000 times before the heat is undetectable.

Regardless of which pepper you choose, the hottest parts are the seeds and white membrane. If you split the pepper open and remove these spicy components, you will naturally tame the wild spirit of the chile.

Whenever you are cooking with hot peppers, say from the jalapeno on up the scale, it's a good idea to wear gloves. When slicing peppers and touching the seeds and membranes with your bare hands, you will get a burn that feels like you've touched a very hot pan. Also, if you touch your eyes or other sensitive body parts (men take note!) within a couple of days after you've chopped the chiles, the serious sting will remind you to go and buy gloves for next time.

As for eating exceptionally spicy foods, having a big glass of water or a cold beer nearby isn't really going to help your cause. The spicy heat is oil based and water does nothing to dilute oil. A better solution is milk or yogurt. Dairy substances will dilute the oil and help wash it down your throat and out of your mouth. Try a Mixed Berry Lassi, a yogurt-based drink served in parts of India and Asia to temper the hot food and the hot weather.  Other natural heat-offsetting sides include rice and tortillas.

So which is it - chili or chile? Often these spellings are used interchangeably. Common acceptable practice in food writing is that "chili" refers to a meat and bean stew while "chile" refers to a hot pepper.

Below is a compendium of fairly common chiles and other items and where they fit on the Scoville scale. Note that color is not an indication of heat; most chiles start out green and turn red, yellow or orange as they mature. You will find, however, that size matters where chiles are concerned. Generally speaking, the smaller the chile the hotter the flesh.  Have a look and see where your favorite peppers rank.

Zero:

Bell pepper - While of the same capsicum species as hot peppers, these sweet peppers don't produce capsaicin, which is the substance that makes peppers hot.  One red bell pepper has about 3 times the amount of vitamin C as your average orange. 

100 to 900:

Peperoncini - These Italian sweet peppers are often pickled and sold in jars. They exhibit a mild heat and a hint of bitterness.

1,000 to 2,500:

Poblano - From dark green in color to almost black (the darker the chile the richer the flavor) poblanos are very mild but can have a little heat. They are about 4 to 5-inches long and 3-inches wide at the stem. Used for making Chile Relleno.

Dried form: Ancho or mulato (smoked and dried and used to make mole)    

Anaheim - These long, slender green chiles are similar to a poblano pepper in heat. They are grown both in California and in New Mexico; the New Mexico chiles tend to be almost twice as hot as the California variety with a heat index of 4,500 to 5,000.

3,500 to 8,000:

Jalapeno - The most commonly used chile in Mexico. They are dark green and turn red when ripe and are about 3 to 4-inches in length. They vary in heat from hot to very hot.

Dried form: Chipotle chiles are Jalapenos that have been smoked and dried.

Tabasco Sauce - A piquant sauce made from Tabasco peppers that are aged for three years and then mixed with vinegar and salt.  

10,000 to 25,000:

Serrano - These small chiles (about 1 1/2 inches long) are very hot. The skin is bright green but turns red then yellow as it matures.

Dried form: Chile seco (found in powdered form as well as whole)

Aleppo - The burgundy pods of the Aleppo pepper are dried and ground and commonly used in Turkish and Syrian cooking. The pepper is named after an ancient town in Syria along the Silk Road. The flavor is complex and slightly fruity, but with a substantial kick. If you can't find Aleppo pepper, an acceptable substitute is a mix of 3 parts paprika to 1 part cayenne.

30,000 to 50,000:

Cayenne - Used whole in some Asian cuisines, the cayenne pepper is more commonly used in dried form in Western cooking, both as a powder and as dried flakes. It is also used extensively in herbal medicine.

Powdered form: Red pepper, red pepper flakes

Tabasco Peppers - These small, red (when ripe) hot peppers are used in the production of Tabasco Sauce and named after the Mexican state of Tabasco. Until recently all peppers used to make Tabasco Sauce were grown on Avery Island, Louisiana. Production has now spread to Central and South America where the climate is more forgiving and a large supply of peppers can be virtually guaranteed each year.

50,000 to 100,000:

Bird's Eye - Also known as Thai chiles, these small, usually red chiles are fiery hot. They are commonly used in Southeast Asian cuisines and can also be found in some regional Indian dishes.  

100,000 to 350,000:

Habañero - The largest consumer of this super hot wonder is Mexico, and the habañero is an integral part of the cuisine on the Yucatan Peninsula. The Scotch Bonnet pepper is of the same species and is often thought to have a similar flavor and heat intensity. The slightly fruity, citrusy flavor has helped to increase the popularity of this chile pepper throughout the world.

855,000 to 1,450,000:

Bhut Jolokia - Also known as the Ghost chile, this pepper was once hailed the hottest pepper in the world. Since then other peppers, such as the Naga Viper and Trinidad Scorpian Butch T, have out-scored the Ghost. In addition to food uses, the chile is used as a deterrent to wild elephants. When preparing the chile for cooking, cooks wear not only protective gloves but usually face masks as well.
  
1,500,000 to 2,000,000:

Most law enforcement pepper spray rates this high on the Scoville index, although it is usually diluted to 200,000 for retail use.

15,000,000

Pure capsaicin. Do not attempt!