We use Exclamations to express surprise, anger, joy and so on:
What an idiot!
Unbelievable!
How fantastic!
As you can see, in writing an exclamation is shown using an Exclamation Mark (or Exclamation Point) which is a vertical line over a dot. See Exclamation Mark for more on this.
An exclamation then is often an interjection – something suddenly expressed to show emotion.
Exclamation Phrases
There are a few main forms of exclamation phrases which are common in English:
{what} + {non-countable noun phrase}
What luck!
{what} + {article} + {countable noun phrase}
What a lunatic!
{how} + {adjective}
How incredible!
How curious!
When we use a question word (what, how, etc) in an exclamation, we do not invert the subject and verb as we would in a normal question:
Normal Question: How big is it?
Exclamation: Look how big it is!