GRADES AND WHAT THEY MEAN
Grade A prospects are the elite. They have a good chance of becoming stars or superstars. Almost all Grade A prospects develop into major league regulars, if injuries or other problems don't intervene. Note that is a major "if" in some cases.
Grade B prospects have a good chance to enjoy successful careers. Some will develop into stars, some will not. Most end up spending several years in the majors, at the very least in a marginal role.
Grade C prospects are the most common type. These are guys who have something positive going for them, but who may have a question mark or three, or who are just too far away from the majors to get an accurate feel for. A few Grade C guys, especially at the lower levels, do develop into stars. Many end up as role players or bench guys. Some don't make it at all.
A major point to remember is that grades for pitchers do NOT correspond directly to grades for hitters. Many Grade A pitching prospects fail to develop, often due to injuries. Some Grade C pitching prospects turn out much better than expected.
Also note that there is diversity within each category. I'm a tough grader; Grade C+ is actually good praise coming from me, and some C+ prospects turn out very well indeed.
Finally, keep in mind that all grades are shorthand. You have to read the full comment in the book for my full opinion about a player, the letter grade only tells you so much. A Grade C prospect in rookie ball could end up being very impressive, while a Grade C prospect in Triple-A is likely just a future role player.