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.

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.

You can find a list of mild interjections, with examples of their usage and punctuation, at the English Club.