The Oxford comma (also called the series comma or the Harvard comma) is the comma used immediately before a coordinating conjunction (usually and or or) preceding the final item in a list of three or more.
with the Oxford comma: I visited Ontario, Quebec, and Alberta.
without the Oxford comma: I visited Ontario, Quebec and Alberta.
Who cares, right? I mean, why do we need the Oxford comma?
Here's why. Use of the comma makes things less ambiguous. Like this book dedication by Teresa Nielsen Hayden:
To my parents, Ayn Rand and God.
wow. Did you know her parents were Ayn Rand and God? Or maybe it really should be:
To my parents, Ayn Rand, and God.
Well done, Oxford comma! You've saved the day again!
And take a look at this fabulous example:
So I'm going to keep using the Oxford comma, because I don't ever want to end up with JFK and Stalin as strippers.