The punctuation mark at the end of a sentence indicates a complete thought. Between the first letter of the first word of the sentence and the punctuation mark at the end of the sentence, the reader can expect to find both a subject and a predicate. The subject of the sentence is the doer of the action that is represented in the predicate.
Sentences can end with a period, question mark or exclamation point. Periods are used to end declarative sentences, or those that make a statement, and imperative sentences, or those that either ask or tell someone to do something. Question marks end interrogative sentences, or those that ask questions. Exclamation points end exclamatory sentences, or those that communicate an urgent message, such as an emotion.
Punctuation marks at the end of sentences let the reader know to pause before moving on to the next sentence, especially when reading aloud. The type of punctuation also tells the reader with what kind of inflection or emphasis she should read the sentence. For example, a reader will use a different inflection or tone when reading a sentence that ends with a question mark than a sentence that ends with a period. In this way, punctuation helps writing mimic spoken language. Most people naturally change their inflection and tone when asking a question. The question mark indicates the reader must do so when reading. Punctuation inside of sentences, such as commas, also helps writing mimic speech. For example, a comma is a signal to pause between words.
Without punctuation marks, writing would be a collection of words listed one after the other. Punctuation marks give meaning to these groups of words by organizing them into thoughts, statements, questions and expressions of feelings.