A fragment resembles a sentence in two ways. Both groups of words begin with a capital letter and conclude with an end mark—usually a period [.] but sometimes a question mark [?] or an exclamation point [!].
The one important difference is that a fragment does not contain a main clause. Like an engine, the main clause powers a complete sentence, propelling the reader through the development of an idea. A fragment, missing this essential component, stalls on the page.