All complete sentences share three characteristics begin with a capital letter, include an end mark and contain at least one main clause. Without each of these components, what would otherwise be a sentence is considered a fragment or an incomplete sentence.
The end mark in a sentence can be a period, exclamation mark or question mark. A main clause contains a subject and a verb and expresses a complete thought, meaning that a sentence can be as short as "I am." Sentences without either a subject or a verb are known as fragments and are considered a major error in writing.