This example runs together two independent clauses, the essay was good and the details support the thesis well, in one sentence, producing a run on. Because each of these clauses has a subject, a verb, and makes sense by itself, the two clauses should be separated by either a period or a semi-colon.

Correction #1: The essay was good. The details support the thesis well.
Correction #2: The essay was good; the details support the thesis well.

Students are sometimes afraid to write long sentences for fear of creating a run on, and the problems they create out of fear are worst than the run on that they hoped to avoid. Whether a sentence is a run on has nothing to do with its length. "I came I saw I conquered" is short, but it is a run on because the two-word independent clauses aren't separated by the appropriate punctuation.

Check the following resources for useful practice material: 1) Guide to Grammar and Writing, 2) Colleen Hildebrand's UCI Correction Symbols with hyperlinks.

For more information on run ons and fragments, consult the LARC Electronic Writing Center Grammar Checklist.