A run-on sentence is not simply a long sentence. Long sentences may be well balanced and perfectly grammatical, as the sentence from Dickens' Hard Times discussed on the Myths page illustrates. At a minimum, a sentence in English contains one clause - that is, a group of words possessing a subject and a predicate. When a clause can stand by itself as a complete sentence, it is said to be independent. Independent clauses must be joined by either a conjunction or appropriate punctuation. (Conjunctions are words such as and, but, yet, although).
Most run-ons occur when a writer joins independent clauses with no conjunction or punctuation, or with a comma (see comma splice). In general, independent clauses joined by punctuation require either a colon or a semicolon. They can also, of course, be written as two sentences, each beginning with a capital letter and ending with a period. The following is an example of a run-on sentence. It contains two independent clauses. It also contains two subordinate (aka "dependent") clauses. (A subordinate clause has a subject and a predicate but could not stand alone as a sentence.) The subordinate clauses appear below in boldface.
In his Marriage-a-la-Mode series, Hogarth critiques the loveless marriages created by the impoverished nobility and the ambitious bourgeoisie, each of whom seeks to satisfy their needs in a partner, in Hogarth's series the bourgeoisie is represented by the wife and the nobility by the husband, who ultimately commits suicide.
The writer has used a comma (following partner) to join the first independent-subordinate combination to the second. In order to correct the error, the writer could (1) replace the comma with a semicolon or conjunction (and would do) or (2) begin a new sentence after partner.
Neither solution, however, would leave the reader with a very clear sense of logical relationships. Editing run-ons offers the writer an opportunity to eliminate wordiness and enhance directness. Compare the original to the following revision:
In his Marriage-a-la-Mode series, Hogarth critiques the loveless unions of impoverished noblemen to daughters of ambitious bourgeois merchants. Although the marriage of convenience satisfies the couple's desire for money and prestige, the nobleman's temporary debt relief cannot prevent his eventual suicide.