Using a comma to separate items in a series.

A series is a list of three or more items. A conjunction (and, or) joins the last two items in the series.
Examples: A. Mom offered to make pizza, hamburgers, or hot dogs for dinner.
B. I like swimming, hiking, and fishing when I am at camp.
C. At the mall, we bought shoes, tried on dresses, and ate pretzels.
Notice that there is a comma after the item that precedes the conjunction.