Originally used by Oxford University Press, this is the comma which is used before and in a list. British English doesn’t use the Oxford comma. It certainly has its purpose when used to eliminate confusion, but it is used consistently – whether it’s required for clarity or not.
American English insists on the use of the Oxford comma.
The American flag is red, white, and blue.
Margaret, Martha, and Martin went to the movies.
The cat leapt onto the chair, turned around several times, and curled himself into a small ball with his tail over his nose.