The Oxford comma, also known as the serial comma, is the comma that precedes the last item in a list of three or more items (e.g., the red, white, and blue). Newspapers (among other sinners) eschew the serial comma for reasons too arcane to pursue here. Thoughtful counselors advise its use. (See, for example, Wilson Follett’s Modern American Usage at pp. 422-423.) Why? Because the serial comma, like the hyphen in a compound adjective, averts ambiguity. It isn’t always necessary, but if it is used consistently, ambiguity can be avoided.
Here’s a great example, from the Wikipedia article linked to in the first sentence of this paragraph: “To my parents, Ayn Rand and God”. The writer means, of course, “To my parents, Ayn Rand, and God”.
Kylee Zempel has much more to say in her essay, “Using the Oxford Comma Is a Sign of Grace and Clarity“. It is, indeed.
(For much more about writing, see my page “Writing: A Guide“.)