This is simply the weight of the diamond when you place it on a diamond scale.  The carat weight of the diamond often does not correspond to the physical size of the diamond, which is also dependent on the cut.  However, because there is only one way to cut round diamonds, rounds are awarded a cut grade by the GIA so you can look at the millimeter diameter of a round diamond and correctly assume its general weight but the same is not true for all other diamond shapes.

The GIA grades white diamond color on a scale of “D to Z”, with D color being “colorless” and Z color being “dark warm yellow tint”.  D, E, and F color are generally grouped together, as the color difference between them is negligible.  G, H, I, and J color are generally grouped together as being “near colorless”, as it is difficult to pick up any warm color when viewing these diamonds “face up”, and can be grouped together in sets of neighbors G-H, H-I, and I-J, as having negligible color difference between them.  Warm color can really start to be seen face up in K color down to Z color.

Diamonds are created deep within the Earth, carbon which has been crystalized under tremendous pressure and heat.  During this formation process there can be features which are left within the diamond.  These features are called “inclusions”, because they are a part of the diamond - included within it.  They are not flaws and do not effect the integrity of the diamond.  The GIA grades diamond clarity on a scale of FL down to I2.  FL (Flawless) and IF (Internally Flawless) mean there are no inclusions within the diamond.  VVS1 and VVS2 (Very Very Slightly Included 1-2) mean there are next to no inclusions, and could require a microscope in the lab to see them.  VS1 and VS2 (Very Slightly Included 1-2) mean there are very few inclusions within the diamond and they require the aid of a loupe to view them.  SI1 snd SI2 (Slightly Included 1-2) mean that there is a slight chance, much less so with SI1 and more so with SI2, that an inclusion can be seen with the naked eye.  And I1 and I2 (Included 1-2) means that there are heavy inclusions throughout the diamond.

In a well cut diamond, light enters through the flat area at its top (called the table), bounces around the hall-of-mirrors-like faceting in its middle (called the pavilion), and returns back out through the table producing the fire, brilliance, and scintillation of an attractive diamond.  A diamond being cut too deep or too shallow will have a negative effect on the light return, resulting in glassiness or darkness.  We seek diamonds which are cut slightly more shallow but still retain their beauty, resulting in seeing more of the carat weight being spread across the top of the diamond instead of being kept down in the pavilion.