An interjection is a part of speech which is added to a sentence to convey an emotion or a sentiment such as surprise, disgust, joy, excitement, or enthusiasm. An interjection is a word or expression used to express strong emotions or feelings. It is normally followed by an exclamation mark (!).
Example: Wow! I won the lottery!
Interjections are words or phrases used to exclaim or protest or command. They sometimes stand by themselves, but they are often contained within larger structures.
An interjection can be followed by either a comma or an exclamation mark. A comma is used for a mild interjection, while an exclamation mark is used for a more abrupt display of surprise, emotion, or deep feeling.
Hurry! The bus is about to leave!
No, I'm not going tomorrow night.
Very often (as in the first example above), an interjection with an exclamation mark is followed by a sentence with an exclamation mark.
An interjection is not grammatically related to any other part of the sentence. It does not interact with any other words in the sentence. It does not modify anything. It does not get modified by anything and it does not play the role of subject or verb. Interjections are rarely used in formal or academic writing.