When you write a list of three or more items, unless you display them as a bulleted or numbered list, you conventionally separate the items with either a comma or the word ‘and’ (or sometimes ‘or’). You may choose to separate the last two items in the list with just the word ‘and’, or with a comma followed by ‘and’:
- Her shirt was red, green and blue.
- Her shirt was red, green, and blue.
If you choose the second option, you’re using the Oxford comma, also called the serial comma. This little punctuation mark arouses surprisingly strong emotions: many people will be happy to tell you that omitting the Oxford comma is simply wrong. Others will assert it is both unnecessary and ugly.