The 4 C's are the basic criteria used to judge the beauty and price/value of a diamond. Most often, diamond shoppers look to color and clarity as the two most important factors when choosing a diamond, however the cut of the diamond will influence fire and brilliance much more than the other two C's.
The way a diamond is cut, that is, the amount and location of the rough diamond that is removed in the cutting process has the most direct effect on how the diamond will look
Graded on a letter scale from D to Z (D being the best, known as colorless), color describes the overall tint, or lack thereof, of the diamond, from colorless/white to yellow
Graded on a scale from FL (Flawless) to I3 (Many Visible Inclusions), clarity describes the amount and location of naturally occuring "inclusions" found in nearly all diamonds
Commonly confused with size, carat refers to the weight of the diamond when measured on a scale. A diamond that weighs more is not necessarily bigger than a diamond that weighs less