The most popular system used to sort pearls is from AAA to A, where AAA is the highest sort. It is usually used to do an estimation of the freshwater pearl, but is also used by some companies producing Tahitian and South Sea pearl.
AAA - High quality pearl jewelry, high luster, it is nearly perfect. At least 95% of the surface is free of the defects of any kind.
AA - High or medium luster or 75% of the surface is defect-free.
A - The low quality pearl, low luster or more than 25% of the surface has defects.
A side note. When buying the pearl, keep in mind that in the piece of jewelry, the pearl will most likely be installed in such a way, that the defects are hidden. This is the way of getting a nice piece of jewelry at lower price. But it is also the way of getting a higher price, if the seller is not honest, or if he simply is not aware of defects.
Do not hesitate to ask for a magnifying glass and do an inspection yourself. A good, respectable seller will allow you to do it, providing (maybe) a cup of coffee and a discount, if you find a defect.
Some sellers use additional, intermediate sorts, to make the scale more flexible. For example, the A+ sort for A pearl that are almost in AA category. Obviously, this system is more subjective, but it is normal in the industry. If you are asked to pay more for AA+, then AA should cost, you may refuse, but you may agree as well, as it says that the sorting have been done with more strict criteria and with more attention.
Just keep in mind that AAA+ does not exist.
Also keep in mind, that for the jewelry containing large number of pearls, the sort is usually not specified, or if it is, it applies to the MOST pearls, not to all of them. It is possible, that few pearls have lower sort, due to the fact, that pearls making a perfect set is more important.