You may have heard the term "Oxford Comma" and thought to yourself, what is that crap all about? Here it is.
The Oxford Comma is the last comma in a list and is placed right before the conjunction. Consider the following sentence: Tom went to Austria, Poland, and Belgium. The comma after "Poland" is the Oxford, also called serial, comma.
But Stu, why should we use it? Hipsters never do and that's considered "grammatically correct!"
I'll give you a few great examples. What if your list isn't of perfectly related items? Consider the following:
Johann came to the party with the two hookers, Washington and Jefferson. No Oxford comma. Still grammatically correct. See the problem? Johann might have shown up with 2 people or he might have brought 4. If he brought 2, he will be introducing you to two hookers, of which their names are Washington and Jefferson. Uh oh. If he brought 4 people, two of them happen to be hookers and the other two are esteemed presidents of the United States. Sweet.
Consider that same sentence WITH the Oxford comma: Johann came to the party with the two hookers, Washington, and Jefferson. That comma makes the sentence very clear. Under no interpretation did Johann screw up and bring hookers named Washington and Jefferson. Get ready to have a pretty fun party.