An interjection expresses an emotion. It might show excitement or surprise.
Wow! That is a giant pumpkin!
Ouch, you stepped on my toe!
Yippee! We won!
Whoa! Hold your horses!
Bravo, you did a great job!
An interjection often appears at the beginning of a sentence. It is usually followed by an exclamation point or a comma.
See also: Interjections: Zap! Pow! Wow!.