Serial commas are commas that separate items in a series and in particular pertain to the use of a comma with the last item listed. Many people ignore this rule, and I’m pretty sure it’s standard policy (to ignore) this rule in AP (article writing) style. which doesn’t make sense to me. It’s very important to always use a serial comma.
The example often used to show the need for the serial comma is this line: “I’d like to thank my parents, God and Mother Teresa, for inspiring me.” Well, by not using the serial comma between God and Mother Teresa, you can see how the meaning of this sentence gets really wacky. I mean–who in their right mind would claim their parents are God and Mother Teresa? The way to punctuate this correctly so as to avoid such a weird interpretation is “I’d like to thank my parents, God, and Mother Teresa for inspiring me.”
Entire books have been written on this topic (see Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation for a great book that stresses the need to be careful with those tiny bits of punctuation).
So, whenever you have a list, be sure you use a comma after each item in that list. You don’t necessarily need one after the very last item—that depends upon the phrase to follow, but we won’t get into that in this post.