Q:

Is it necessary to use commas in lists before the conjunction?

I have seen many sentences like this:

"Capitalism, Socialism, and Anarchism"

and this:

"She was happy, and cheerful"

Do we have to use a comma before "and", or is it incorrect?  If it is not, then why?

A:

For two items, the use of a comma between elements of a list is forbidden

I would like fish and chips
(X) I would like fish, and chips

For lists of three or more items, the choice of whether a comma before the final and (called the Oxford comma, Serial comma or Harvard comma) is contentious. Some people believe it should be mandatory - others forbid its use.
In my experience, Americans and Canadians tend to err on the side of requiring its use; British and Australian English tends to err on not using it.
Some people argue that the use (or omission) of the serial comma can cause ambiguities. Both are right, as shown below:
Ambiguities arising from omission of the serial comma

This book is dedicated to my parents, The Queen and God.

This sentence is ambiguous between:

The comma is merely separating the three elements "my parents", "The Queen" and "God" and this book is dedicated to all three
The comma is used to denote apposition, meaning that this book is dedicated to my parents, who (by the way) are "the Queen" and "God".

Ambiguities arising from use of the serial comma

This book is dedicated to my mother, The Queen, and God

This sentence is ambiguous between

The comma denotes a list of the three elements as before and
The two commas again denote apposition to suggest that the book is dedicated to my mother (who by the way is the Queen), and also the book is also dedicated to God.

From the Wikipedia article:
The Chicago Manual of Style, Strunk and White's Elements of Style, the United States Government Printing Office, most authorities on American English and Canadian English, and some authorities on British English (for example, Oxford University Press and Fowler's Modern English Usage) recommend or require the use of the serial comma.
Newspaper style guides (such as those published by The New York Times, The Los Angeles Times, the Associated Press, The Times newspaper in the United Kingdom, and the Canadian Press) recommend against it.
The differences of opinion on the use of the serial comma are well characterized by Lynne Truss in her popularized style guide Eats, Shoots & Leaves:

There are people who embrace the Oxford comma, and people who don't, and I'll just say this: never get between these people when drink has been taken.

A:

As an ESL, I understood the Oxford/Serial/Harvard comma with this example:

Since then I've been careful.

