For more about commas see our Punctuation Overview page.
When a sentence contains a list of items, it is usual to put a comma after each item in the list except for between the last two items – this place usually has the word ‘and’.
I went to the zoo yesterday where I saw lions, tigers, monkeys, zebras and snakes.
My favourite tea is sausage, egg, chips and beans.
Sometimes it is better to put a comma before the ‘and’ in a list of items. This is particularly true when there might be some confusion about the meaning.
For instance in this sentence:
My favourite sandwiches are cheese, ham, salad and bacon and egg.
It might be unclear whether the writer likes his bacon and egg in the same sandwich or in separate sandwiches. By putting in a comma after the penultimate item, it becomes dear that the writer intends ‘bacon and egg’ to be a single sandwich, thus:
My favourite sandwiches cheese, ham, salad, and bacon and egg.
The final comma in these lists, before the word ‘and’, is called the ‘serial comma’.
Not every writer uses a comma in this position; it is optional. It is also known as the ‘Oxford’ Comma because Oxford dictionaries always use it.
We can also, of course, use lists of adjectives to describe a noun; when we do this, the adjectives are separated by commas. The commas go between the adjectives. Note there is no comma between the last adjective in the list and the noun that follows it.
Red Riding Hood was afraid of the big, bad, ugly, snarling wolf.
More about adjectives here… Adjectives Overview