Interjections are short exclamations that convey an emotion or a sentiment such as joy, grief, dismay, surprise, shock, contempt, applause or enthusiasm. An interjection has no grammatical connection to the sentence. Also, an interjection is sometimes followed by an exclamation mark (!) when written.
A few examples are as follows:
Oh, that’s a surprise.
Geez! That was close.
Introductory expressions such as ‘yes,’ ‘no,’ ‘indeed’ and ‘well’ are also classified as interjections. For example:
Well, I don’t know much about that.
Some interjections indicate a pause, such as: ‘um,’ ‘hmm’.
Interjections like ‘er’ and ‘um’ are also known as ‘hesitation devices.’ They are extremely common in English. People use them when they don't know what to say, or to indicate that they are thinking about what to say. We normally learn to recognize them when we hear them and realize that they have no real meaning.
Most mild interjections are treated as parenthetical elements and set off from the rest of the sentence with a comma or set of commas. If the interjection is more forceful, however, it is followed with an exclamation mark. Interjections are rarely used in formal or academic writing.