To copy editors, the “serial comma” refers to separation of listed elements (in a series, get it?). Specifically, the final comma in the construction “I like apples, pears, and oranges.” Somewhere along the line, newspaper editors who were obsessed with saving space decided the final comma was redundant. Today, the serial comma is omitted more often than not.
This, IMHO, is a real shame. Not only does the serial comma more accurately reflect speech (“apples … pears … and oranges,” as opposed to “apples … pearsandoranges”), it avoids confusion in many constructions. Consider as just one example:
Mary, John and Bill are at work.
Are three people at work, or is a speaker telling Mary that two people are at work?
Obviously, context comes to the rescue in this instance. But what about something like:
Our office edits books and magazines, designs posters and flyers and prints postcards and stamps.
A comma just doesn't take up that much space. Use the serial comma and spare your readers from confusion. Here endeth the sermon.
UPDATE: Read in the New York Times how a missing serial comma cost one Maine company $10 million.