These express the extent to which speakers are impressed or aroused by something. The are often short sentences or even one word. Although, they can take the form of major sentences too.
Their first element begins with what or how, and is followed by a subject and verb, in that order. For example:
What an awful night that was!
What a masterpiece you've made!
How swell she is!
They can offer appear in a reduced form.
What an awful night!
What a masterpiece!
Both sets of examples are said to have an exclamative structure.
It's rare to find an exclamative with the subject and verb inverted, but they do exist. For example:
How often have I cursed that terrible day!