Cut indicates the angles and proportions of the diamonds.
A well-cut diamond will present the most brilliance and "fire". The proportions between the depth and the table of a well-cut stone will create this beautiful reflection.
Diamonds that are cut too deep or too shallow will loose the light from the side or the bottom and will result in losing the brilliance and the value of the stone.
The Round diamond is the cut that has perfect symmetry and therefore has the most brilliance from all other "fancy shapes" (Princess, Emerald, Pear..etc..)
The "Fancy Shapes" will have their own guidelines to be considered well-cut. Its mostly a question of personal taste.
75% of the diamonds sold in the world are the round shape because the cut is the most symmetrical and the most brilliant.
