I'm sure you've heard the debates surrounding the use of the serial, or Oxford, comma. No? What, do you live under a rock? Do you have more important things to do than sit around, wondering how to perfectly construct a typed list? Eh, I know. It seems unimportant. Here's why it's not:
The office just won the lottery! We must divide our winnings evenly between accounting, human resources, marketing and research.
So, how small is your pot getting here? How many departments get a cut in this lottery win? Is marketing and research one department or two? This is a sentence without an Oxford comma. Compare that to this:
The office just won the lottery! We must divide our winnings evenly between accounting, human resources, marketing, and research.
Now how many divisions must you make?
Some people argue that, because punctuation was originally developed to mimic the nuances of spoken language, a comma is unnecessary when proceded by the word "and." It is argued that "and" represents enough of a pause, therefore rendering the comma redundant. But what about clarity? This is why the use of the Oxford comma is such a hot topic amongst copyeditors, whose cardinal rule is to improve a work's level of correctness, consistency, and clarity.
The truth of it is that in most cases the Oxford comma is optional as long as its use or omission is done with consistency throughout a piece. There are other times, however, when a comma can have a larger impact on or even change the entire meaning of a sentence. Consider these two statements:
The ball belongs to the dog, that has just arrived from the pound.
The ball belongs to the dog that has just arrived from the pound.
In the first example, there is only one dog---the information about where he has come from is supplementary. In the second example, there are multiple dogs and the information about the pound is used to identify one particular dog---the lucky sod who owns a ball.
Here's a fun example:
I like to eat, my grandpa, and my pets.
I like to eat my grandpa and my pets.
Or how about:
Let's eat, Grandpa!
Let's eat Grandpa!
So you see, comma placement IS a big deal. A comma can change the way a sentence is read, and it can even change the meaning of the words. If used correctly, a comma can assist in clarity of language. If used incorrectly, well ... make sure Grandpa is at the table, too, before you begin eating.