The ‘Oxford comma‘ (aka the serial comma or the Harvard comma) is an optional comma before the word ‘and’ at the end of a list: We sell books, videos, and magazines. It’s known as the Oxford comma because it was traditionally used by printers, readers, and editors at Oxford University Press.
I use it almost every time in a series, unless it is quite clear that there is no confusion as to whether the last two items are related.
I enjoy movies, plays, Netflix and books. (no need)
For breakfast I had eggs, bacon, toast and jelly. (correct)
For breakfast I had eggs, bacon, toast, and jelly. (not wrong, but not necessary)
Those are four items, but toast and jelly are considered one item to most people, so it doesn’t matter whether you use it here. People will understand.
I like many flavors of ice cream: chocolate, mint, vanilla, and peaches and cream.
To put “…vanilla, peaches, and cream.” is incorrect, because peaches and cream is the name of the flavor.
There are arguments for and against the comma, which usually come down to comprehension. Wikipedia quotes these ambiguities:
My parents, Mother Teresa and the Pope. (2 people)
My parents, Mother Teresa, and the Pope. (4 people)
Most people might know that Mother Teresa and the Pope are not your parents, but that comma helps clarify.
As in a dedication: To my parents, Bob and Sue, and Julie. (this could be 5 or 3 people)
It is not clear whether Bob and Sue are the parents. To work around that say,
To my parents – Bob and Sue – and Julie. (3 people) or:
My parents, Bob, Sue, and Julie (5 people). But if you are simply listing people, use it as a serial comma (a list).
Put a comma before the concluding conjunction in a series, however, if an integral element of the series requires a conjunction: (these are three elements)
Those going on the mission trip must be knowledgeable about the area, be skillful in their trades, and possess the mental capacity for endurance.