Interjections are words added to a sentence to express emotion. For example, in "Yikes, that was scary," the interjection is yikes. Interjections have no grammatical relationship to the rest of the sentence (which results in an interesting floating sentence diagram, for those of us who care about such things).
All kinds of exclamations are interjections, ranging from ah-ha to zowie. Curse words are interjections. Introductory words like no, yes, well, and indeed are interjections too. Sounds can also be interjections, such as phew, hmm, and ah. Here’s a list of common interjections.
How you punctuate an interjection depends on how much emotion you want to convey. Use a comma for mild feeling. An exclamation mark shows much more emotion. Examples: "Well, I’m not going" OR "No! I’m not going."
While fiction writers often use interjections in dialogue, writers in business, academic, or technical fields do not, unless they’re quoting from a passage that includes the interjection.
Whether you liven up a sentence with jeepers or hallelujah, interjections bring feeling to your writing.