The comma -- that tiny little backwards "C" that means "brief pause here" -- is one of the most easily understood and most misused elements of punctuation in the English language. If a mistake surfaces in an otherwise perfect manuscript, chances are that it's comma-related.
This is unfortunate, because using commas is not all that difficult. In all honesty, if you simply read the manuscript aloud and place commas everwhere you naturally pause, their placement and usage will generally be correct.
The comma separates clauses. It keeps every time frame and sub-topic within a sentence neatly corralled and contained:
Suddenly, I noticed that the room, which was decorated in execrable shades of lime, was intolerably warm.
Commas surround names when making introductions:
My assistant, Igor, entered the lab.
They separate the various elements of days and dates:
It all began on Thursday, January 9, 1969.
And they separate the elements of a series:
I wish the contents of my estate to be divided equally among Tom, Dick, and Harry.
Writers often drop the comma before the last element of a series. They will write that the flag is "red, white and blue," rather than "red, white, and blue." Ah, but legally, this can cause no end of consternation.
If a person's will states:
I wish the contents of my estate to be divided equally among Tom, Dick and Harry,
Tom will then get half of everything. Dick and Harry (poor saps) will split what's left between them. Dick and Harry, therefore, will each get a quarter of the whole.
If commas fall after "Tom" and "Dick," then the estate will be divided into thirds, and none of the boys will receive more than the others.
The lawyers have figured out the importance of using commas correctly. It behooves us as writers to do the same.