The serial comma, or the Oxford comma as it is popularly known, is the comma before the conjunction at the end of a list. In British English, using it is optional. However, most style guidelines mark this as mandatory in the US. It is certainly preferable, especially in academic writing, to use the serial comma for the sake of clarity and economy.
She took a photograph of her parents, the president and the vice president. [No serial comma results in some ambiguity. Did she take a photograph of two people i.e., her parents, who are also the president and the vice president?]
She took a photograph of her parents, the president, and the vice president. [Serial comma used: clarity retained. She took a photograph of 4 people.]
The table was covered with gifts, food and flowers. [What are the gifts? Food and flowers]
The table was covered with gifts, food, and flowers. [On the table, there were gifts, and there were flowers, and food.]