The Oxford comma is an (optional!) comma which can be placed before placed before the "and" or "or" in a series of nouns. It is fiercely defended by (some) grammarians on the grounds that it can change the meaning of a sentence.
Comparing these two sentences clarifies the difference:
a) The greatest influences in my life are my sisters, Oprah Winfrey and Madonna.
b) The greatest influences in my life are my sisters, Oprah Winfrey, and Madonna.
Taken literally, the first sentence could mean that Oprah and Madonna are sisters. This sensational interpretation is prevented by the Oxford comma in the second sentence.
So Oxford comma enthusiasts are correct?
An alternative view would be that such examples of potential confusion are vanishingly rare. Opponents of the Oxford comma consider it to be fussy, unsightly and/or pedantic.
A third position would be to make the Oxford comma optional - but this would raise eyebrows with most stylebook editors.