Also known as the Oxford comma, it’s the one that follows the next to last item in a list. For examples we can look at newspapers, any website, and content providers. (See, in that construction, you don’t get confused about whether someone is providing content as well as websites). Print media, where I learned my practices, isn’t fond of the serial comma, because journalism is literature in a hurry.
But when you leave that comma out, unexpected calamity or hilarity can ensue. One favorite t-shirt reads
“Let’s eat, Grandma.”
“Let’s eat Grandma.”
Commas save lives!
Now comes this example from Sky News (the UK version of Fox News) demonstrating the need for a serial comma. Certainly would be an interesting wedding between world political figures, though.