Cut Grade is one of the Four C’s.
This is the assessment of the quality of the cut of a diamond. Since about the year 2000, major labs began assigning official grading of the cut, which completely changed the way that consumers can compare the beauty of one diamond to another. The Cut Grade is an excellent tool for the consumer, because it takes out the guesswork, and makes it unnecessary (or much less necessary) to learn all the Proportions and Measurements of a diamond and what they mean.
The Cut Grade is a much more accurate and far reaching consideration of the overall beauty and Light Performance in a diamond, and is based on the science of observation and calculation of actual flow of light, rather than guessing how a diamond will look. It considers how everything works together, which cannot be truly calculated from the singular stats and measurements we used to use. Always look at the official Cut Grade on the Cert / Certificate when you are comparing any two diamonds.
Here is a link to the AGS web page about their version of Cut Grade.
Here is a video that explains more about the GIA Cut Grade ::
Here is an interactive from GIA that helps you understand their Cut Grade (note there are three different slides for each of the Cut Grades, to show possible variations) ::