What are run-ons?

Answer 1

In respect to standard written English, run-on sentences are defined as compound sentences lacking proper punctuation. A compound sentence may consist of two or more independent clauses; however, these independent clauses must be connected with the punctuation marks appropriate for the construction in question. While each writer of a compound sentence may choose the punctuation marks deemed most appropriate, the absence of punctuation marks will in all cases render the sentence as a "run-on".

Answer 2

A run-on sentence is a sentence in which two or more independent clauses (that is, complete sentences) are joined without appropriate punctuation or conjunction.