The GIA clarity scale includes eleven clarity grades ranging from Flawless to Included 3. Diamonds occur naturally and are formed under the Earth's crust, so it is extremely rare to find a diamond that lacks internal inclusions and external clarity characteristics. These byproducts of their formation help gemologists separate natural diamonds from synthetics and simulants. They are also used to identify individual stones. (They're like a diamond's fingerprint)
The fewer inclusions in a diamond, the more rare and therefore the more valuable the diamond. However, diamonds with a clarity grade of SI1 (Slightly Included 1) and above have inclusions that are nearly impossible to detect without the aid of magnification. So, the SI clarity range is a great way to get a diamond that looks great to the naked eye, and also saves a little.