Grade of "A": A thorough and thoughtful treatment of the assigned topic presented in a logical and convincing manner, or, in a research paper, an original argument presented in a logical and convincing manner. The A essay will have a clearly articulated thesis that leaves no doubt as to what the writer thinks and why. The ideas are original and complex. In a research paper the student will carefully and appropriately use sources to support her/his own argument. The writing is not hampered by grammatical or stylistic problems, but is notable for its clarity and verbal fluency.
Grade of "B": A pretty good paper in most ways, but generally less thoughtful than A work. Often B papers are those which mostly parrot back what the professor and readings have taught, but do so in a way that makes it apparent that the student understands the concepts, but just doesn't add much to them. In a research paper, a student may clearly develop a topic rather than a thesis. The writing style may be less fluid or sophisticated than the A paper's, but it is still quite competent and easily readable.
Grade of "C": An essay that shows an understanding of most of the concepts involved in the assignment, or, a research paper that may identify a topic or thesis, but doesn't thoroughly treat them or doesn't synthesize them in an entirely satisfactory manner. Whether in an assigned-topic paper, or in a research paper, its thesis is usually vague; if the thesis is clear, perhaps it is contradicted by arguments or evidence in the essay itself. In a research paper, it may show a tendency to "over-quote" other authors. The writing is comprehensible but often awkward due to grammatical errors that make understanding it an effort.
Grade of "D": Seriously flawed. The student neither demonstrates understanding of the material nor articulates any coherent argument about it. The paper might wander among several ideas without developing any single one. There will not be a thesis in this kind of work. In a research paper a student might rely on quotations and others' work, rather than developing her/his own ideas. The teacher might wonder if the student even tried at all.
Grade of "F": Little redemptive appears in F work.
The paper fails to address the assignment in fundamental ways. There
is no real answer to any of the problems posed by the assignment, and there
will be no real engagement of the topic in any way. The writing often
consists of "stream-of-consciousness" prose that fails to cohere at all.
It's rarely necessary to give F's.