A tooth abscess differs from a gum abscess by the source of the original infection. The tooth abscess (or periapical abscess) originates from the pulp of the tooth and exits out the tooth's apex at the bottom of the root. A gum abscess (or periodontal abscess) starts in a gum pocket outside of the tooth next to the root.
Click here to see a 239-word sentence that is a perfectly fine sentence (structurally). When two independent clauses are connected by only a comma, they constitute a run-on sentence that is called a comma-splice. The example just above (about the sunscreen) is a comma-splice.
(wrong) It's easy to see in that example why the error is called a comma splice: it's because the comma is used to splice together two complete sentences when that isn't the function of a comma. Commas aren't meant to join main clauses all by themselves; to force them into that role is to perpetrate a comma splice.