The comma (,) is a punctuation mark used to separate information within a sentence. Commas are used to separate the following:

Three or more items in a list
Complete thoughts joined by a coordinating conjunction
A complete thought from its introduction
Non-essential information

Lists

When making lists of at least three items, a comma should follow each item before the final item listed.

Examples

Add the second substance, stir, and record any observations.
Today’s ceremonies will consist of a procession, a greeting, a keynote speech, and a reception.

When deciding whether or not to put a comma before the and that comes before the last item in a list, the answer is one of preference: a comma may or may not be included.

Serial comma

Although the final comma (called a serial comma or Oxford comma) is not always required, adding the final comma can be a helpful habit to develop. In certain situations, the serial comma may avoid confusion for the reader.

For example, the following sentence (without the serial comma) can be understood two ways:“I went to the beach with my parents, Bill and Marie.”

I went to the beach with two people: my parents, whose names are Bill and Marie.
I went to the beach with four people: both of my parents, a friend named Bill, and another friend named Marie.

A serial comma would clarify to the reader that I went to the beach with four people instead of being ambiguous.

Separating independent clauses

When a sentence contains two independent clauses linked by a coordinating conjunction, a comma is usually placed before the coordinating conjunction.
The seven coordinating conjunctions are and, but, for, or, nor, so, and yet.
An easy test to see if a comma is required is to count the subjects before and after the coordinating conjunction, as each independent clause includes its own subject.

Examples

The article includes many examples and is well written.
(This sentence contains only one subject article, so it does not require a comma.)
The planter class did not want to lose its free source of labour, but the abolitionists were determined to demolish slavery.
(There is a subject before and after the coordinating conjunction but, so a comma is required.)

Separating introductory information

A comma is used to distinguish between the main idea in a sentence and words or phrases that help introduce the main idea. Introductory information includes transitional expressions (such as in addition or consequently) and dependent clauses.

Examples

There are flaws in postmodern discourse. However, elements of postmodernism could be used to help minority groups gain equal rights.
Despite his regular absence from the domestic sphere, Ramsay is a continuous presence in the thoughts of his wife and children.

The comma in each example above clearly separates the main idea of the sentence (the independent clause) from its introduction.

Separating non-essential information

Sentences often include words or phrases with information that is not essential to a reader’s understanding. Using commas is a way to separate this information from the rest of a sentence.

Examples

Philadelphia, an important landmark of American history, is also known as “The City of Brotherly Love.”
The irony of this, however, is that Mina acts with her own brand of vampirism.

As the examples demonstrate, two commas are used to indicate non-essential information (shown as underlined).