With a comma splice, two independent clauses have merely a comma between them, again without a conjunction—e.g.: “I need to go to the store, the baby needs some diapers.”
The presence or absence of a comma—and therefore the distinction between a run-on sentence and a comma splice—isn’t usually noteworthy. So most writers class the two problems together as run-on sentences.
But the distinction can be helpful in differentiating between the wholly unacceptable (true run-on sentences) and the usually-but-not-always unacceptable (comma splices). That is, most usage authorities accept comma splices when (1) the clauses are short and closely related, (2) there is no danger of a MISCUE, and (3) the context is informal. Thus: “Jane likes him, I don’t.” But even when all three criteria are met, some readers are likely to object. And in any event a dash seems preferable to a comma in a sentence like that one.
—A Dictionary of Modern American Usage, Bryan A. Garner.