You can read our in-depth explanation of the diamond 4Cs here.
Carat is a unit of weight, not size, so measurements can vary.
Colors of diamonds range from D (Colorless) to Z (Colored). Diamonds in the G-J range are considered "near colorless", meaning it is difficult to detect color in them but there can be a slight warmth in the diamond. Colorless diamonds (ranging from D to F) are extremely rare, so most diamonds on today's market range in the G-J range. The color metal you plan to set the diamond in is important, as "I" colored stones may have some warmth and are recommended and preferred in Yellow and Rose Gold settings, so the warmth in the setting can reflect off of the slight tint in the diamond.
Clarity refers to a diamond's inclusions. Inclusions are very slight imperfections in the diamond's interior that are nearly invisible to the naked eye and typically require 10x magnification to view. Diamonds that are VS2+ will be eye-clean, with imperfections only visible under 10x magnification.
Finally, cut refers to how the stone was polished to reflect the most light and brilliance - aka, the diamond's sparkle. Ideal is the best we offer, meaning it meets exact proportions to reflect the most light. Only Round Brilliants can be graded as Ideal, because their proportions have been perfected by empirical and mathematical data, and have the strictest cut standards out of all the shapes. Excellent is a step below that, and then Very Good and Good.