The “Oxford Comma” is also known as the “serial comma.” This punctuation mark is placed immediately before the coordinating conjunction (usually “and,” “or,” or “nor”) in a series of three or more terms.

The necessity of the Oxford Comma is a hotly debated topic. Some dismiss it as overly pretentious or merely unnecessary. Using an Oxford Comma in a series, however, generally helps to provide consistency and clarity, and avoid strange misunderstandings.

For example: I love my parents, Frank Sinatra and Dolly Parton.

Without the Oxford Comma, the sentence above could be interpreted as stating that you love your parents — and your parents are Frank Sinatra and Dolly Parton. Having a single comma here creates a situation known as the appositive. An appositive is a noun or noun phrase that restates another noun or noun phrase that preceded it. Most appositives are set off with commas. In other words, it looks like the last two items in this list (Frank Sinatra and Dolly Parton) are appositives of “parents” instead of independent items in the list.

Here’s the same sentence WITH the Oxford Comma: 
“I love my parents, Frank Sinatra, and Dolly Parton.”

With the Oxford Comma after Sinatra, it is clear that this is a series of three things the subject loves.

Some writing style guides necessitate the Oxford Comma; others, like the AP guide, omit it (though the omission is explained primarily as a space saver in print). Whether you choose to use the Oxford Comma or not, the most important thing is to be consistent in your writing. Pick a side, follow a style guide, and write!