What is the Oxford Comma?

In English-language punctuation, a serial comma (also called a series comma, Oxford comma, or Harvard comma) is a comma placed immediately after the penultimate term (i.e., before the coordinating conjunction, such as and or or) in a series of three or more terms. For example, a list of three countries might be punctuated either as "France, Italy and Spain" (without the serial comma) or "France, Italy, and Spain" (with the serial comma).