A comma before the “and” at the end of a list is known as a serial comma or Oxford comma.
Commas are used to separate each item in a list (or series), and the last item is separated from the others by a conjunction (and, or). A serial comma before the conjunction is optional. US English tends to use it and UK English tends to avoid it but, whether or not you choose to use it, make sure to be consistent.
|With serial comma||Without serial comma|
|He bought milk, bread, and a box of bananas.||He bought milk, bread and a box of bananas.|
|She studied English, French, and Zulu.||She studied English, French, and Zulu.|
Sometimes a serial comma is necessary for clarity. One example is when the final list item itself contains a conjunction.
Confusing: People are often afraid of heights, crowds and thunder and lightning.
Clear: People are often afraid of heights, crowds, and thunder and lightning.
Using the serial comma here clarifies that the final item, business and leisure, is a single category. Without the serial comma, the list is confusing.
Another example is when items at the end of the list could be read as describing an earlier item.
Confusing: He had to take care of two dogs, Jane and Louis.
Clear: He had to take care of two dogs, Jane, and Louis.
Here the serial comma clarifies that two dogs, Jane, and Louis are separate items. Without the serial comma, the sentence seems to say that he had to take care of two dogs named Jane and Louis.
A list of phrases or clauses is always clearer if there is a comma before the final item.
Confusing: They counted crows, read their guidebooks and reminded each other where to go.
Clear: They counted crows, read their guidebooks, and reminded each other where to go.