To earn an "A," work needs to be be completed thoroughly, with all directions followed and answers/performance with specificificty (i.e., citing text evidence). When work is a "B," it is good but not excellent. Both show an understanding of learning and high work quality.
A "C" is average; a "D" is below average. Both, usually lack some of the above traits of proficiency. An "F" is poor quality, usually incomplete work. These show a lack of understanding, effort, details, and a lack of direction following.