A comma is a punctuation mark that indicates a pause in a sentence or separates items in a list. A comma is also used before the words "and" or "but" to join two independent clauses.
Commas come in handy when you're writing a grocery list: "Buy candy, chocolate milk, Pop Tarts, and gum." You can also use a comma to set off an adverb at the beginning of a sentence: "Therefore, I refuse to ride the roller coaster." The word comma has been around since the 1500s, and in Latin in means "short phrase," from the Greek komma, "clause in a sentence," or literally, "piece which is cut off."