Interjections are words that express excitement or emotion, such as "uh", "hey", and "yikes". They are followed by punctuation like commas or exclamation points. While inappropriate for formal writing, interjections are commonly used in dialogue to show a character's reaction. Examples of interjections used in dialogue are "argh!", "eek!", and "hmm...". Interjections help convey emotion and should only be included in writing when directly quoting speech.