A - An exceptional game. To earn an "A" a game must have either done something innovative or totally nailed it. You should pay full retail, buy the tee shirt, pass the word around and do everything you can to encourage the developer to do another.
B - A well made professional game. This game should leave you wanting more. This game deserves a place on your shelf. You should support the developer and pay full retail for it.
C - A playable game, but with issues. While this is a passing grade, there are defects which make the game appear less than professional. This is a game which could tide you over until the next "A" or "B" game comes out. Wait for it to go on sale.
D - Unplayable. This game is just bad and should be avoided. If you are a collector who must have a copy of every game made, then get it at a garage sale. In no way should any of your money get back to the developer.
F - So bad it's camp. This is a game which is so bad that it is actually funny. This is a game which they would feature on MST3K. Consider it a guilty pleasure.
+/- simply mean that the game barely made it to that grade. With just a little improvement would have made the next tier (+) or be comfortably within its grade (-).