A serial comma is a comma placed just before a coordinating conjunction, such as "and" or "or," in a list. For example, the list "apples, oranges, and pears" contains the serial comma while "apples, oranges and pears" does not. Placing a comma before the coordinating conjunction is a particularly common practice in American English, though it is often omitted in newspaper articles. It is used less commonly in British English and is actually considered grammatically incorrect in some non-English languages. This comma is also, at times, referred to as the Oxford comma, the Harvard comma, or the series comma.
The use of the serial comma can lead to ambiguity in some cases but can actually clear up ambiguity in others. Problems with ambiguity usually arise because of similarities to appositive phrases, as it may be difficult to tell for sure if the writer is adding a new item to the list or adding detail to a previous item. The list "my brother, Andy, and Dave" is ambiguous because "Andy" may be the brother's name or may be an entirely different person. There is, however, no ambiguity when the list is presented as "my brother, Andy and Dave," as commas would be used on both sides of "Andy" if the name were, indeed, an appositive.
Omitting the serial comma is not always less ambiguous and can, in fact, also lead to ambiguity in some situations. If the above list were changed to "my brothers, Andy and Dave," ambiguity now exists because "Andy and Dave" might be the brothers or they might be two people other than the brothers. Formulating the list as "my brothers, Andy, and Dave" resolves this ambiguity as the serial comma indicates that the structure is a list and not an appositive phrase. When either use or omission of the serial comma can cause clear ambiguity, it tends to be better to follow the structure that does not, in fact, make the list difficult to understand.
In many cases, the choice to use or omit the serial comma simply comes down to personal preference. Some style guides say it's better to use it while others say it's better to omit it. When writing for a particular publication, however, it is generally necessary to subscribe to the style guide followed by all of the publication's writers. As such, it is important to check with style guides or editors about comma usage before submitting written work for publication.