It is a word or a phrase which expresses some sudden feeling or emotion like grief, joy, anger, surprise etc.
Thus, we may say that a word or a phrase used as sudden exclamation can be called an interjection. Interjection is always followed by a sign of exclamation.
For Example:
How nice of you to come here!
What a pleasant surprise!
Such an intelligent class!
How stupid you are!
Joy: Hurrah! Huzzah!
Hurrah! We have achieved our goal.
Sorrow/Grief: Alas! Alack!
Alas! I have missed the train.
Approval: Bravo! Buck-up! Well done!
Bravo! You have done well.
Reproof: Shame!
Shame! You have failed badly.
Ridicule: Pooh! Pish!
Pooh! You cannot beat me.
Call: Ho! Hey!
Hey! Come here.
Doubt: Hum! Hem!
Hum! I do not know anything about it.