An interjection simply expresses an emotion as a reaction to something. Example:
Wow! That’s amazing.
Ouch! That hurt.
In the above examples, the words wow and ouch express the speaker’s reaction to an event, the first offering surprise, the second representing pain.
How to use interjections
As interjections are spoken, they are inherently informal, so they should be used sparingly if at all in more formal prose. However, when they are used, it is important to consider the context and punctuation. Example:
I was expecting a big crowd but, wow, this is incredible.
The use of commas places the interjection neatly in the sentence. The positioning of the word is not really important, as long as the context is clear. If it was a standalone word, it would probably (but this is not obligatory) be used with an exclamation mark. Example:
Wow! I was expecting a big crowd, but this is incredible.