An exclamative or exclamatory sentence is released because of, and expresses strong emotion. They many times feel like involuntary reactions to a situation, yet they can technically be stifled if need be. And while exclamatives most usually manifest themselves as one or two word interjections, they can also come as major sentences. They are essentially unfiltered vocalizations of our feelings, and a form of self-talk because they are directed either at the speaker themself or at nobody in particular. In punctuation, an exclamative is ended with an exclamation mark.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/exclamative-sentence