The scale is based on a very complicated procedure of diluting the pepper sample in sugar water until the heat can't be tasted anymore. I'd get into it more, but you wouldn't understand me (cause I'm smerter then you).
This is the basic Scoville scale:
Bell peppers, sweet italian peppers
Thai, pequin, chiltepin
Red Savina Habanero
The bell pepper starts off the scale, coming in at a measly zero. In actuality, bell & sweet peppers can get as hot as 150 Scovilles! Wow (he exclaimed sarcastically), keep those away from your eyes...
Bell's come in several colors including red, yellow, orange & green, but they all have the same Scoville rating just above zero. How exciting... still, they have a purpose and they're great when you grill them up on a shishkebob or add them to just about any (shudder) salad.
As you can see, you can barely even consider a pepper mild until you reach poblano peppers which can reside between 2,000 & 3,000 Scovilles.