Three modes of thought exist for serial commas: always use them, never use them, and only use them to avoid confusion. My advice, as is the advice of any serious writer, is to always use the serial comma. The only folks who do not use the serial comma are journalists following the AP Stylebook.
In a list of 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, these arguments fall 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 possibility: using the serial comma only in situations that otherwise lead to confusion. But, as she points out, this is an inconsistent approach and may make writers appear uncertain in their grasp of usage and style. Always use the serial comma.
Some call the serial comma the Oxford comma—named such due to its use by the style guide of the Oxford University Press—or, for the Americans, Harvard comma. What you call it is up to you.