The Comma

There are some general rules which you can apply when using the comma.
Using the comma to separate phrases, words, or clauses in lists
A series of phrases

On my anniversary I went to the club, ate dinner in a restaurant, and went dancing.

Use a comma + a little conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses.
One of the most frequent errors in comma usage is the placement of a comma after a coordinating conjunction.

He hit the ball well, but he ran toward third base.

A series of nouns

The meal consisted of soup, fish, chicken, dessert and coffee.

Use a comma to set off introductory elements.

Running toward third base, he suddenly realized how stupid he looked.

Use a comma to set off parenthetical elements

The Founders Bridge, which spans the Connecticut River, is falling down.

When both a city's name and that city's state or country's name are mentioned together, the state or country's name is treated as a parenthetical element.

We visited Hartford, Connecticut, last summer.
Paris, France, is sometimes called "The City of Lights."

When the state becomes a possessive form, this rule is no longer followed:

Hartford, Connecticut's investment in the insurance industry is well known.

Also, when the state or country's name becomes part of a compound structure, the second comma is dropped:

Heublein, a Hartford, Connecticut-based company, is moving to another state.

A series of adjectives

She was young, beautiful, kind, and intelligent.
Note: if an adjective is modifying another adjective you do not separate them with a

e.g. She bought a bright red shirt.
A series of verbs

Tony ran towards me, fell, yelled, and fainted.

Use a comma to set off quoted elements. Because we don't use quoted material all the time, even when writing, this is probably the most difficult rule to remember in comma usage. It is a good idea to find a page from an article that uses several quotations, photocopy that page, and keep it in front of you as a model when you're writing. Generally, use a comma to separate quoted material from the rest of the sentence that explains or introduces the quotation:

Summing up this argument, Peter Coveney writes, "The purpose and strength of the romantic image of the child had been above all to establish a relation between childhood and adult consciousness."

If an attribution of a quoted element comes in the middle of the quotation, two commas will be required. But be careful not to create a comma splice in so doing.

"The question is," said Alice, "whether you can make words mean so many things."
"I should like to buy an egg, please," she said timidly. "How do you sell them?"

Be careful not to use commas to set off quoted elements introduced by the word that or quoted elements that are embedded in a larger structure:

Peter Coveney writes that "[t]he purpose and strength of . . ."
We often say "Sorry" when we don't really mean it.

And, instead of a comma, use a colon to set off explanatory or introductory language from a quoted element that is either very formal or long (especially if it's longer than one sentence):

Peter Coveney had this to say about the nineteenth-century's use of children
in fiction: "The purpose and strength of . . . .”

A series of clauses

The car smashed into the wall, flipped onto its roof, slid along the road, and finally stopped against a tree.

Using the comma to enclose insertions or comments. The comma is placed on either side of the insertion.

China, one of the most powerful nations on Earth, has a huge population.

Use the comma to mark off a participial phrase

Hearing that her father was in hospital, Jane left work immediately.

Use the comma in 'tag questions'

She lives in Paris, doesn't she?
We haven't met, have we?

Use to mark off interjections like 'please', 'thank you', 'yes', and 'no'

Yes, I will stay a little longer, thank you.

Use commas to set off phrases that express contrast.

Some say the world will end in ice, not fire.
It was her money, not her charm or personality that first attracted him.
The puppies were cute, but very messy.

Never use only one comma between a subject and its verb. "Believing completely and positively in oneself is essential for success."
Typographical Reasons: Between a city and a state [Hartford, Connecticut], a date and the year [June 15, 1997], a name and a title when the title comes after the name [Bob Downey, Professor of English], in long numbers [5,456,783 and $14,682], etc. Although you will often see a comma between a name and suffix — Bob Downey, Jr., Richard Harrison, III — this comma is no longer regarded as necessary by most copy editors, and some individuals — such as Martin Luther King Jr. — never used a comma there at all.
General notes:
1. Misplacing a comma can lose friends!
Putting a comma in the wrong place can lead to a sentence with a completely different meaning, look at these two examples:

I detest liars like you, I believe that honesty is the best policy.
I detest liars; like you, I believe that honesty is the best policy.