The Scoville scale is a unit of measure to help define the hotness of any given chili pepper, as determined by the amount of capsaicin (the chemical compound that makes peppers hot) present. The scale is named after its creator, an American chemist named Wilbur Scoville. He developed a test for rating the heat or pungency of chili peppers. His method is precisely called the Scoville Organoleptic Test.
In Scoville's method, a solution of the pepper extract is diluted in sugar syrup until the "heat" is no longer measurable or detectable to a panel of tasters - generally five people. The degree of dilution this gives is how the Scoville scale gets it's meaure. A sweet pepper or a bell pepper has no capsaicin at all, so it has a Scoville rating of zero, meaning no heat detectable, even undiluted. On the other end of the heat spectrum, the hottest chilis, such as habaneros, have a rating of 200,000 or more, indicating that their extract has to be diluted 200,000-to-1 before the capsaicin is undetectable. Since this test relies on human subjectivity and taste buds, the Scoville Organoleptic Test is at best imprecise.
That having been said, here's some peppers and their Scoville ratings.