It's a pausing separator! That's it. The comma is probably one of the most over used punctuation marks scribed in the existence of language history. When using the comma there are a few rules you can glean from to make sure you do not plethorate this hesitating divider throughout a sentence unnecessarily.
Here are the rules -
1: Use the comma to divide elements in a series. - The quarterback ran to the mark, cocked back his arm, and threw the football like a bullet. She grabbed her coat, gloves, hat, umbrella and cigarettes.
2: Use the comma before quotation marks. Sally asked Roger, "Do these shoes go with this dress?"
3: Use the comma inside quotation marks. "Was that it," as Roger always asks after watching a movie thriller.
4: Use the comma before conjunctions. She was terribly upset, but she recovered after he gave her flowers. So, this is the way things are going to be. ( but, and, nor, yet, for, or, so)
Here is a little resource page with good information on comma use:
Make sure when you edit to check your comma's usage. If you think, "should a little pause go here," it probably needs a comma. However, you have to be careful because over using comma's can make a sentence unnaturally flow. Believe me, when most people read what you write they are not checking for your structuring comma use. No one really cares if you put an extra comma here or there, but it is important to know when to use a comma properly.
To use, or not use the comma, this is the question!