Almost all diamonds contain naturally occurring internal characteristics called inclusions. The size, number, and character of the inclusions is used to determine its official grade. Clarity is often misunderstood, however, as many inclusions that effect value, are in fact not visible to the naked eye, and may not effect brilliance and beauty. Each stone is different, and you cannot rely solely on a grade to make your selection.
Clarity Grades:

FL - Flawless. No internal or external flaws

IF - Internally Flawless. No internal flaws.

VVS1, VVS2 - Very, Very Slight Inclusions - Inclusions are very difficult to see under a 10x jeweler’s magnifying loupe

VS1, VS2 - Very Slight Inclusions - Inclusions are difficult to see under a 10x jeweler’s magnifying loupe

SI1, SI2, SI3 - Slight Includions - Inclusions easy to see under a 10x jeweler’s magnifying loupe

I1, I2, I3 - Inclusions that are visible to the naked eye.