A sentence fragment is an incomplete sentence. Sometimes a fragment is missing a subject and/or a verb. Other times, a fragment has a subject and a verb, but the thought still isn’t complete.
A run-on sentence, on the other hand, is a sentence that has too many complete thoughts or ideas that need to be further broken up, either with punctuation, transition words, or a mix of both.
Identifying the subjects, verbs, and supplemental parts of a sentence is the first step to being able to recognize sentence fragments or run-on sentences. See the handouts linked below for specific examples.