This is a confusing element for a whole lot of people. Most content I see (like 90%) gets this wrong. I’m not sure why, because it’s really much easier to understand when you use it. But, most people just don’t want to put that comma in before the conjunction. 

The Chicago Manual of Style, with just a few exceptions, says to use serial commas for clarity. “Items in a series are normally separated by commas … When a conjunction joins the last two elements in a series, a comma – known as the serial or series comma or the Oxford comma – should appear before the conjunction. Chicago strongly recommends this widely practiced usage … since it prevents ambiguity.” CMS 6.19

The Associated Press Stylebook basically says to use the serial comma in most cases except for a simple series: My face was covered in black, blue and yellow bruises. (This is how most people do ALL their series – too much newspaper reading?) Everything else needs a comma before the conjunction.

Strunk and White, The Elements of Style states except in cases such as business names, “In a series of three or more terms with a single conjunction, use a comma after each term except the last.” Using an example from this excerpt: gold, silver, or copper.

This is as basic as I can get. I would recommend just putting the commas in there for any kind of listing, because it does make things a whole lot cleaner.

When in doubt, it’s better to serial than not to serial.

Examples:

Johnny packed the truck full of yellow ribbons, purple boxes, and orange fences.

I went to the store, did some laundry, and screamed out the window at the old lady picking my roses.