independent clause A word group with a subject and a predicate that can stand alone as a separate sentence.
coordinating conjunction A word that joins comparable and equally important sentence elements: for, and, or, but, nor, yet, or so.
When independent clauses are joined by a coordinating conjunction, a comma is required to tell the reader that another independent clause follows the first one.
If the independent clauses are brief and unambiguous, a comma is not required, though it is never wrong to include it.