In a run-on, two complete sentences have been joined together incorrectly and punctuated as though they were a single sentence. Here is an example of a run-on:
This kind of run-on is called a comma splice because it incorrectly uses a comma to join two complete sentences. If the two sentences had been put together without any punctuation at all, it would be another kind of run-on called a fused sentence. Writers sometimes create run-ons when they try to keep closely related ideas together within the same sentence. Two good ways to achieve the same goal are to join the related sentences with a comma and a coordinating conjunction (for, and, nor, but, or, yet, so) or with a semicolon (;).