What is the Oxford comma?
The Oxford comma is an (optional!) comma which can be placed before 'and' at the end of a list. It is fiercely disputed amongst grammarians and was the cause of a 'punctuation war' in a Milwaukee law office:
Tom Kamenick battled fellow attorneys at a Milwaukee, Wis., public-interest law firm over use of "the Oxford comma"—an additional comma placed before the "and" or "or" in a series of nouns. Leaving it out can change the meaning of a sentence, Mr. Kamenick says: The sentence, "The greatest influences in my life are my sisters, Oprah Winfrey and Madonna," means something different from the sentence, "The greatest influences in my life are my sisters, Oprah Winfrey, and Madonna," he says. (The first sentence implies the writer has two celebrity sisters; the second says the sisters and the stars are different individuals.)More here