The serial comma (also known as the Oxford or Harvard comma) is the comma placed before a coordinating conjunction (usually and and or, sometimes nor) that precedes the final item in a list of three or more items:
“I visited Norway, France, and Germany.”
The Chicago Manual of Style says to always use it. The Associated Press Stylebook, however, says you do not need it in a simple series (“I like the colors red, blue and yellow.”) but you do in a complex series (“While in Barcelona, we ate at three tapas restaurants, walked down Las Ramblas, and ran along the beach.”).
The trend in professional writing over the last decade or so has been: “the less punctuation, the better.” So you will see some articles in major publications that to not have a serial commas in a complex series, do not use commas to offset a non-restrictive clause beginning with which, do not have a comma before too when it comes at the end of a sentence, and other uses (or non-uses) of punctuation which appear to be errors.
So what’s the final verdict? Use the serial comma at your own discretion, that is, follow your Writer’s Intuition. The reason often quoted for using the serial comma is to prevent sentences like this one which refers to Merle Haggard and appeared in a major newspaper:
“Among those interviewed were his two ex-wives, Kris Kristofferson and Robert Duvall.”
However, common sense and your Writer’s Intuition tells you such a sentence needs a comma before the and to clear up the confusion. So put one in. But your Writer’s Intuition will tell you the serial comma is not necessary in other cases. For example, “Among those interviewed were Kris Kristofferson, Robert Duvall and Haggard’s two ex-wives.”