Grade A is smooth with no knots. It is strong and often used for furniture, like bookshelves, because of its attractive appearance.
Grade B is almost as solid as grade A. It has a few knots and other very slight imperfections.
Grade C may have more blemishes than grades A and B, including splits and discoloration. It is also easier to shape because it isn't as strong.
Grade C Plugged
C plugged is slightly higher than grade C. It has smaller cracks and splits, but it is basically the same strength-wise.
Grade D plywood has large knots and holes. It is certainly not strong like grade A and breaks down easily when it gets wet. Therefore, it should not be used for anything outdoors.
Plywood is usually described by more than one grade. AA grade is smooth on both sides and used for nice furniture. AC grade is used for flooring because it is nice on the front and ugly on the back.