Up the garden path − or, why a fanboy comma is your friend
The comma is a valuable punctuation device because it separates the structural elements of sentences into manageable segments.
Use commas to separate independent clauses (that is, a part of a sentence that can stand on its own) when they are joined by any of these seven coordinating conjunctions: for, and, nor, but, or, yet, so. The acronym of this list of conjunctions is FANBOYS, so these commas are known as FANBOYS commas.
The game was over, and she was exhausted.
The sentence above is short and clear, so the comma is not really necessary. But in longer sentences, this comma really helps to avoid ambiguity:
Without the FANBOY comma:
The study noticed a general reluctance to relinquish control of those services already delivered locally and criticism of externally provided services tended towards the hypercritical.
Without the comma, this sentence could easily read as:
The study noticed … general reluctance … and criticism.
This is being ‘led up the garden path’. A reader should be able to stay with the author of a sentence for the whole trip, without having to double back and re-read to make sense out of the material. In the above example, the reader has to wait until they get to ‘tended’ to realise that ‘criticism’ is actually the beginning of the second clause, not the end of the first one.
With the FANBOY comma:
The study noticed a general reluctance to relinquish control of those services already delivered locally, and criticism of externally provided services tended towards the hypercritical.
Now it’s clearer that the sentence is really two sentences:
1. The study noticed … general reluctance.
2. Criticism … tended towards the hypercritical.
The comma alerts the reader that, in effect, a new sentence is starting. The reader will process ‘criticism’ as the first word in the new sentence, not as the second item in the list of things noticed in the study.
There’s a comma called the Oxford, or serial comma, which comes before the last ‘and’ in a list:
The government works for voters, non-voters, and donkey voters.
There is no clear grammatical rule about whether to use this comma or not – it is a question of style.
In Australian style this comma is usually not used, except where it is required to avoid ambiguity:1
They should seek the support of landholders, philanthropists, government, and community and industry groups.
Here, ‘community and industry groups’ functions as the last item in the list. A comma has been placed before this last item to ensure that ‘community’ stays in the reader’s mind as an adjective for ‘groups’.
The serial comma is used more in American style than in British style (except, of course, at Oxford University Press, after which it is named), but there is by no means consensus. Wikipedia2 has a detailed explanation of who prescribes and proscribes its use, as well as many examples of how it is used to avoid ambiguity.
 Snooks and Co. 2002. Style manual: For authors, editors and printers, 6th Edn. John Wiley & Sons, Australia. p. 102.