The serial comma is the final comma between a list of items. A serial comma-ist may write something like: I went to the store and got bread, apples, and eggs.
A non serial comma-ist would write: I went to the store and got bread, apples and eggs.
Serial-comma activists insist that the serial comma is necessary to avoid ambiguities such as this: I dedicate this book to my parents, Ayn Rand and God. Which implies that your parents are Ayn Rand and God.
However, I have noticed that my use of the serial comma on my website creates ambiguity: Courtney Milan lives in the Pacific Northwest with her husband, a marginally-trained dog, and an attack cat. Which rather implies that my husband is a marginally-trained dog. Of course, this ambiguity can be avoided by switching clause order: Courtney Milan lives in the Pacific Northwest with an attack cat, a marginally-trained dog, and her husband. But I think I’ll keep it the way it is.
Mr. Milan is my favorite marginally trained dog!