The ‘Oxford comma’ or the ‘serial comma’ is an optional comma before the word ‘and’ or ’or’ in a list:
They have a dog, a cat, and a hamster.
Do you have this shirt in black, white, or grey colour?
The reason it is called the Oxford comma is because it was primarily used at the Oxford University Press.
There are lots of arguments about the topic. Some linguists think the Oxford comma is unnecessary, while some believe that it provides clarity and helps avoid misunderstandings.
For example: I love my family, Angelina Jolie and Winona Ryder.
Without the Oxford comma at the end of the list it seems that Angelina Jolie and Winona Ryder are your parents.
I love my family, Angelina Jolie, and Winona Ryder.
It is now clear what you meant.
I love Angelina Jolie, Winona Ryder and my family.
This one is clear as well, even without the Oxford comma.
As it is not obligatory, you probably wonder whether to use the Oxford comma in your writing, or not?
Whatever you decide, try to be consistent in it.