The serial comma is the comma placed before the "and" or "or" in a series (that is, a list) of three or more items:
Nothing could be better, Dean thought, than a bottle of whiskey, a juicy bacon cheeseburger, and a double feature of Star Trek II: The Wrath of Khan and Star Trek IV: The Voyage Home.
Some style guides, however, leave out the serial comma. For a rundown of reasons why you might choose to either use or omit the serial comma, see the original article, "A Very Special Comma," by theemdash.
And for even more info on the serial comma, you can visit Grammar Girl.