[This is a fast-access FAQ excerpt.]
This is known as the "serial comma" dispute. Both styles are
common. The second style was recommended by Fowler, and is Oxford
University Press house style (hence it is also called "the Oxford
comma"; it is also known as "the Harvard comma"); it is more common
in the U.S. than elsewhere. Although either style may cause
ambiguity (in "We considered Miss Roberts for the roles of Marjorie,
David's mother, and Louise", are there two roles or three?), the
style that omits the comma is more likely to do so: "Tom, Peter, and
I went swimming." (Without the comma, one might think that the
sentence was addressed to Tom.) "I ordered sandwiches today. I
ordered turkey, salami, peanut butter and jelly, and roast beef."
Without that last comma, one would have a MIGHTY weird sandwich!
-- Gabe Wiener. James Pierce reports that an author whose custom it
was to omit the comma dedicated a novel: "To my parents, Ayn Rand