Fragment and run-on are both errors in writing sentences, but they’re fundamentally different.
Fragment and run-on are fundamentally different types of error
A sentence fragment is a group of words punctuated as a sentence, even though it’s not a sentence. Strictly speaking, it’s an error because a non-sentence can’t be punctuated as a sentence. In the examples below, the underlined parts are fragments.
The top tennis players earn so much. Yet hanker for more.
They made plans for a weekend getaway. But ended up staying at home.
A run-on sentence is a sentence that results when two sentences are joined just by a comma or by nothing. It’s an error because two sentences can’t be joined like that. When two sentences are joined by a comma, the run-on sentence is called comma splice. And when two sentences are joined by nothing, the run-on sentence is called a fused sentence. Examples:
The police remained tight-lipped, they said investigations were on. [Comma splice]
The police remained tight-lipped they said investigations were on. [Fused sentence]
The difference between fragment and run-on sentence has been expressed succinctly by Paige Wilson and Teresa Glazier in their book The Least You Should Know About English: Writing Skills:
Fragments don’t make complete statements, and run-ons make too many complete statements without punctuating between them.
But unlike run-on sentences, which are rarely considered appropriate, sentence fragments are sometimes desirable.
Fragments can sometimes be used to good effect. Run-on, rarely.
Fragments may have earned bad reputation in formal writing, but they can sometimes be used to good effect. That’s why they’re often used by professional writers. Here are few examples of fragment’s acceptable use:
There is a remedy for everything. Except death.
I am not going to meet him again. Never.
Learn about acceptable uses of fragments:
Run-on sentences, however, rarely qualify as good use.