A sentence fragment is part of a sentence set off as if it were a complete sentence: Because I didn't care. Being an accident. Although he asked again.

Although they're common in speech they should be used sparingly in writing, and only then for specific effects. In fact, sentence fragments should all but entirely be confined to fiction writing. In academic work they are grammatically incorrect and stylistically inappropriate.

Many nineteenth-century horror stories have been made into films. Such as Dracula and Frankenstein.

Many nineteenth-century horror stories have been made into films, such as Dracula and Frankenstein.

A sentence fragment often looks like an afterthought. Perhaps because it was an afterthought.

A sentence fragment often looks like an afterthought -- perhaps because it was an afterthought.