The Oxford Comma
The Oxford comma, also known as the Harvard comma or the serial comma, is the comma preceding the final item in a list (They went to the park, dinner, and a movie). The Chicago Style manual recommends using it. The Associated Press Stylebook -- the guide of choice for HowStuffWorks.com -- recommends against using it. It's mostly a matter of preference for those not writing professionally or academically.
Wrong: The woman took her son, and her nephew to a course on safe driving.
Why: There's a comma before the conjunction joining compound direct objects.
Right: The woman took her son and her nephew to a course on safe driving.
Why: Just like with compound predicates, two compound objects ("son" and "nephew") joined by a conjunction ("and") do not take a comma before (or after) the conjunction. However, three or more compound direct objects would need commas, since they would constitute a list (The woman took her son, her daughter and her nephew to a course on safe driving).
Next, another compound issue …