Capsaicin is the substance that makes hot peppers hot, and the amount of Capsaicin present varies from pepper to pepper.
Chillies naturally produce capsaicin to protect them from being eaten by mammals. Birds are not affected in the same way and can eat even the hottest chilli with no effect.

In 1912, Wilbur Scoville devised a scale to place hot peppers in order of heat. In this highly subjective test, a chili of each variety was placed in a sugar solution and diluted until the heat could no longer be sensed when drunk by a volunteer . The number of dilutions required was assigned an number; the actually pungency of a pepper is measured as multiples of 100 units, where the bell pepper scores a Scoville rating of zero whilst Habanero weighs in at 300,000 Scoville Heat Units. Incidentally, pure Capsaicin is worth 16,000,000 Scoville Units! Nowadays, the Scoville measurement is determined using High Performance Liquid Chromatography.

Here is a chili heat scale measured in scoville units.

Scoville Units
Chile Pepper Heat Range
Sweet Bell 0
Cherry 00 – 500
Pepperoncini 100 – 500
El-Paso 500 – 700
Santa Fe Grande 500 – 750
Coronado 700 – 1 000
Espanola 1 000 – 2 000
Poblano 1 000 – 2 000
Ancho 1 000 – 2 000
Mulato 1 000 – 2 000
Pasilla 1 000 – 2 000
Anaheim 500 – 2 500
Sandia 500 – 2 500
NuMex Big Jim 500 – 2 500
Rocotillo 1 500 – 2 500
Pulla 700 – 3 000
Mirasol 2 500 – 5 000
Guajillo 2 500 – 5 000
Jalapeno 2 500 – 8 000
Chipolte 5 000 – 8 000
Hot Wax 5 000 – 10 000
Hidalgo 6 000 – 17 000
Serrano 8 000 – 22 000
Manzano 12 000 – 30 000
Shipkas 12 000 – 30 000
De Arbol 15 000 – 30 000
Jaloro 30 000 – 50 000
Aji 30 000 – 50 000
Tabasco 30 000 – 50 000
Cayenne 30 000 – 50 000
Santaka 40 000 – 50 000
Super Chile 40 000 – 50 000
Piquin 40 000 – 58 000
Yatsafusa 50 000 – 75 000
Haimen 70 000 – 80 000
Chiltecpin 60 000 – 85 000
Thai 50 000 – 100 000
Tabiche 85 000 – 115 000
Bahamian 95 000 – 110 000
Carolina Cayenne 100 000 – 125 000
Kumataka 125 000 – 150 000
Jamaican Hot 100 000 – 200 000
Birds Eye 100 000 – 225 000
Tepin 100 000 – 265 000
Devil Toung 125 000 – 325 000
Fatalii 125 000 – 325 000
Orange Habanero 150 000 – 325 000
Scotch Bonnet 150 000 – 325 000
Choclate Habanero 300 000 – 425 000
Red Savina Habanero 350 000 – 575 000
Dorset Naga 800 000 – 900 000
Naga Jolokia 800 000 – 1 001 300
Pure Capsaicin
15 000 000