I just came across these web pages and thought others might find them as useful as I have.
Embryo grading system: There doesn't appear to be any standard grading system for embryos -- One clinic might grade on a scale of 1-4 with 1 being the best, while another might grade 1-5 with 5 being the best and so on -- but this page explains the different aspects of the embryo that are looked at when assigning a score.
Blastocyst grading system: This page explains the most commonly used blastocyst grading system and what the numbers and letters refer to. (5AA, 3AB etc.) Full of great info, easy to understand.