There are 5 different types of diamond clarity gradings, these are as follows.
FL -IF Flawless: No internal or external characteristics. Internally Flawless: May have external blemishes only. Both extremely rare.
VVS1-VVS2: Very, very slightly included. Characteristics miniscule and difficult to see under 10x magnification, even to a trained eye.
VS1-VS2: Very slightly included. Minor inclusions ranging from difficult (VS1) to somewhat easy (VS2) to see at 10x magnification.
SI1-SI2: Slightly included. Inclusions noticeable at 10x. Best value if eye-clean. SI2 inclusions may be detectable to a keen unaided eye.
I1: Included. Diamonds may have minor eye-visible inclusions. Blue Nile offers a limited selection of jewellery with I1 clarity diamonds.
I2-I3: Included. Will likely have more obvious or distracting inclusions. If you buy from Blue Nile.com they do not carry I2-3 clarity diamonds.