Consistently, clearly and effectively communicates its purpose to its audience in all areas of writing: consistently clear focus, sufficient development, and coherent in terms of organization and style. The ideas are also well thought out and have significance within the rhetorical context.
A B paper
Is strong in most areas but intermittently deficient in one area or contains minor problems in more than one area. For instance, the essay may be strong in all areas but have some problems with audience, portions may lose focus or be underdeveloped, or there may be some distracting inconsistencies or errors in style (coherence).
A C paper
Generally accomplishes the main job of the assignment--so it maintains its purpose. But it's either intermittently deficient in two categories or consistently deficient in one. For instance, there may be intermittent problems with both audience and development, or the whole essay may be consistently underdeveloped.
A D paper
Is consistently deficient in two areas--for example, consistently unfocused and underdeveloped--to the degree that the deficiencies undermine the purpose of the essay. An unfocused and underdeveloped essay, for instance, would not be able to convey its message to a reader in any significant way. The essay could also have enough serious problems in a combination of areas that the purpose is undermined. It could also miss a major portion of the assignment--like an essay which has no connection to the assigned topic.
An F paper
Is an essay that either was not turned in or is severely deficient in almost all areas. Or it could be an essay that completely fails to address the assignment.