The clarity of diamonds is commonly graded according to the scale developed by the GIA – Gemological Institute of America. This scale assigns diamonds into different scales according to the number of inclusions found inside them. Here is a brief overview of the different clarity grade levels established by the GIA.
FL (Flawless), IF (Internally flawless): these are extremely rare stones, and priced at an extreme premium.
VVS1, VVS2 (Very, very slightly included): these are stones that can be found in most decent jewelry stores, staying on the shelves for years, claimed to be the top piece of the store
VS1, VS2 (Very slightly included): very good quality diamonds at a price reachable for those who are willing to put rather serious money into diamonds
SI1, SI2 (Slightly included): these are diamonds with inclusions noticeable for the aided eye, usually offered at a quite reasonable price
I1, I2, I3 (Included or Imperfect): - these are diamonds with flaws visible with the naked eye, generally not available at jewelers.
Sources
Gemological Institute of America. http://www.gia.edu
Online Diamond Buying Guide. http://onlinediamondbuyingguide.com