The Oxford comma (/ɒksfəd kɒmə/), Harvard comma (/hɑ:vəd kɒmə/), or serial comma (/sɪərɪəl kɒmə/) is the use of a comma in a syndetic list after the penultimate member of that list, just before the co-ordinating conjunctions "and", "or", or "nor".
For example: "apples, bananas, and cherries."
It is traditionally used by the publisher Oxford University Press (OUP).
In theory, it's possible to construct silly ambiguous sentences without using the Oxford comma. In practice however, most writers manage to avoid this.
Some people are big fans of the Oxford comma. Whichever side of the fence you come down on, the best thing is usually consistency. It's an example of the sort of linguistic aspect that native English speakers have pointless arguments about.