Commas reflect pauses in speech.
A listing comma is used to separate items in a series or list. In British English, the last two items in a list are not usually separated by a comma unless these are long.
The Three Musketeers were Athos, Porthos and Aramis.
I went to China, Japan, Malaysia and Singapore.
A joining comma is used to join two complete sentences into a single sentence. It is usually followed by a connecting word like and, or, but, while or yet.
We can go swimming, or we could stay here.
I decided to come home earlier than I had planned, and the others spent the evening at the local disco.
A gapping comma is used to show that certain words have been omitted instead of repeated.
Jane decided to order the home-made steak pie and Alice, the duck special. (The omitted words are decided to order.)
When subordinate clauses begin sentences, they are often separated by commas.
After I left school, I went to London.
If words or expressions interrupt the normal progression of a sentence, we usually separate them off by commas.
John, however, did not turn up.
We were, believe it or not, in love with each other.