It's not often that I write a Grammar Tip post that lets you make the decision, but this just happens to be one of those comma's that isn't always required.
The Sentence: I went to the store and bought eggs, milk, peanut butter and jelly.
It happens to be your choice whether to put in that comma between the last two items in the sentence: peanut butter and jelly. Without the comma, though, the sentence can be interpreted a couple different ways. 1) Peanut butter and jelly were two separate items. 2) Peanut butter and jelly are grouped together for a reason, the same way salt and pepper might be. (Have you ever seen those jars with both the peanut butter and the jelly in one jar? Did you buy that? Then leave off the comma.)
To avoid the different interpretations, place a comma between all items in a series. It just prevents a lot of confusion.
Try another sentence, where the confusion is nastier (borrowed from Grammar Girl): I went to see Zack, an officer and a gentleman.
Here, without the serial comma between an officer and a gentleman the sentence has two very different meanings. Did I go to see three people? (Zack, the officer, the gentleman) Or one person who is both an officer and a gentleman? Without the serial comma, most people will probably read it as the second instance.
I tend to agree with Grammar Girl on this issue (as I do on most grammar issues ...). Using a serial comma in between all the items adds consistency and prevents confusion. So unless you're British or writing for a newspaper and pressed for space, just go ahead and add that last serial comma in; that way you know you'll never be wrong.
To read Grammar Girl's take: http://grammar.quickanddirtytips.com/serial-comma.aspx
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I've been writing since I was old enough to grasp a crayon--my grandma even has an early copy of a "book" I made her. I have a bachelor's degree in history from the University of Wyoming and will (hopefully) soon be starting a graduate program in English. When I'm not breaking up impromptu UFC fights in the living room or losing miserably to my boys at Uno, I'm ... well, writing or editing, of course! I'm married to my best friend, and we have three rambunctious but simply amazing little boys.