The 'Oxford comma' is an optional comma before the word 'and' at the end of a list:
We sell books, videos, and magazines.
It's known as the Oxford comma because it was traditionally used by printers, readers, and editors at Oxford University Press. Not all writers and publishers use it, but it can clarify the meaning of a sentence when the items in a list are not single words:
These items are available in black and white, red and yellow, and blue and green.
The Oxford comma is also known as the 'serial comma'