The Oxford comma (aka the Harvard or serial comma) is a comma put in the list right before the "and" or "or" that connects the last item in the list. For example you could write a list saying "Tom likes to brush his teeth, comb his hair, and shower before going to bed." In that sentence the comma before the and is considered the Oxford comma. I'm sure you will also note that "Tom likes to brush his teeth, comb his hair and shower before going to bed" is also correct. So why is the Oxford comma important? Why does it matter?

The English language is generally a vague language. We can make our language more precise with the use of grammar. A favorite example is: "I invited strippers, Stalin and JFK." The reason that this sentence is vague is because we don't know if the writer invited strippers and Stalin and JFK or if the writer invited two strippers named Stalin and JFK. Those are two completely different meanings.

Below is an infograph from yourdictionary.com.

The Oxford comma is not in every style guide. In fact the AP style guide has the Oxford Comma omitted. Writers will need to refer to the appropriate style guide on whether or not the comma should be included. Whenever writing for everyday matters, on the other hand, it's a good idea to use the Oxford Comma. It allows for natural pausing between listed items. It also helps with spacing between each listed item, visually.

For more information about the Oxford comma, check out these sites: Grammar Girl and MentalFloss.