The serial comma (also known as the Oxford comma or Harvard comma) is the comma used immediately before a conjunction (such as "and" or "or") in a list of three or more items. The phrase "ham, chips, and eggs" is written with a serial comma, but "ham, chips and eggs" is not.
The main justification for the serial comma is to reduce ambiguity, although both its absence and its presence can, in fact, produce it. The use or omission of the serial comma also produces different rhythms in a sentence.
The terms "Oxford comma" and "Harvard comma" come from Oxford University Press and Harvard University Press, where use of the serial comma is the house style.
The Chicago Manual of Style, Strunk and White's Elements of Style, most authorities on American English and Canadian English, and many authorities on British English — for example, Oxford University Press and Fowler's Modern English Usage — recommend the use of the serial comma. Newspaper style guides, such as those published by The New York Times, the Associated Press, and the Canadian Press recommend against it for reasons of space. For this reason the lack of a serial comma is sometimes referred to as the "editorial comma."
In Australia, the United Kingdom, and South Africa, the serial comma tends not to be used in non-academic publications unless its absence produces ambiguity. Many academic publishers (e.g. Cambridge University Press) also avoid it, though some academic publishing houses in these countries do use it. The Australian Government Publishing Service's Style Manual for Authors, Editors and Printers recommends against it.
(From Wikipedia)