A sentence is a sequence of words that expresses a complete thought. It is an independent clause that contains a subject and a verb. (Note: The subject can be implied, as in an imperative: Stop! The implied subject in this case is you.)
We get into trouble when we combine two complete thoughts into one sentence without using the appropriate punctuation or conjunction.
The run-on sentence
A run-on sentence contains two or more independent clauses with no connector between them. If independent clauses are not separated into distinct sentences by a period, then they must be connected by a comma and a conjunction or by a semicolon.
Remember that the length of a sentence does not determine whether the sentence is a run-on. Run-on sentences have structural flaws that can occur even in short sentences.
Long sentences are not necessarily run-on sentences. Here is a 200-word sentence that is not a run-on.
The comma splice
Another type of run-on sentence is a comma splice. A comma splice is a sentence with a comma between the two independent clauses.