Three modes of thought exist for serial commas: always use them, never use them, and use them only to avoid confusion. But I can simplify these to the only one worth following: always use them. The only editors who still struggle to enforce serial commas are those following the The Associated Press Stylebook.
In a list of three or more items, the serial comma precedes the coordinating conjunction: I bought eggs, milk, and ham (the serial comma follows milk). The notion that the serial comma is optional likely arose from the journalistic want to save time, ink, and space. Nowadays, though, with most journalistic material written and printed digitally, such want falls flat. Omitting the serial comma leads to ambiguity, popularly and humorously demonstrated in examples like We invited the strippers, JFK and Stalin, which, without the serial comma, suggests that both JFK and Stalin are the strippers we invited. Mary Norris, in Between You Me: Confessions of a Comma Queen, suggests a third option—to use the serial comma only in situations that otherwise lead to confusion. But, as she also points out, this is an inconsistent approach that may make writers appear uncertain of their grasp of usage and style. In short, always use the serial comma.
Some call the serial comma the Oxford comma—named such for its use by the style guide of the Oxford University Press—or, for Americans, the Harvard comma. Whatever you call it, its behavior remains unchanged.