In a series of three or more items, a comma separates each item in the list. If the last two entries are connected by a conjunction—usually “and” or “o”—a comma usually comes before the conjunction as well. That comma is known as the serial comma.
Associated Press style, however, mostly used by newspapers and magazines, omits the comma before the conjunction, probably to save space. (Obviously, when I write or edit for a publication that uses AP style, I don’t use the serial comma.)
As I’ve written about previously, the serial comma is one of the most contentious pieces of punctuation. Yes, people do argue about such matters, and not just when they are drunk and have finished bickering about Iraq, the Tuck Rule, and whether the American Idol judges are too nasty this year. (Note the use of the serial comma in that last sentence.)
I am in favor of using the serial comma for the simple reason that it reduces ambiguity. Take this example that I heard in an editing class at EEI Communications:
A man died. His will said that his estate “should be split between his sons: Gordon, Andy and Stewart.” The executrix divided the man’s estate evenly between the three men. Gordon, however, sued. He argued that the lack of a comma before “and” meant that the estate should be divided so he got one half of it and his brothers split the other half.
The judge agreed with Gordon; his share went from one-third of the estate to one-half of it. Andy and Stewart went from getting one-third each to one-quarter each.
If the father wanted his estate to be split evenly between his three sons, a serial comma would have ensured that happened. And if he wanted Gordon to have a larger share, specifying that disbursement (“Gordon gets half of my estate; Andy and Stewart each get a quarter of it”) would have made his intentions clearer. (The instructor claimed the story was true.)
If you want more details about arguments for and against using serial commas, read Wikipedia’s entry on the subject.