An interjection is a word or phrase that indicates emotion and is unrelated to the grammatical syntax of the rest of the sentence. Because the interjections are outside of the rest of the sentence, they are usually the easiest of the eight parts of speech to recognize. A word that functions as another part of speech can be an interjection when it stands alone.
Let's take a look at the word "great." In most cases, it is an adjective, as in "a great man," "a great book," "a great vacation." But look at this exchange:
"We're going to a movie."
In the response, the word "Great!" is an interjection because it is a word that stands alone to indicate emotion. Some words like "Wow" or "Oh" are always interjections. (Some grammarians, however, might say that "great" is still an adjective since the response is the implied, "That's great!")
Here is a short list of interjections and the emotion that they usually convey:
Hey! (greeting, or warning, or attempt to get someone's attention)
Yay! (celebration or happiness)
Wow! (surprise or admiration)
Hurrah/Hurray! (celebration or happiness)
Aw...(feeling bad, exaggerated cuteness, appreciation)
Oh! (recognition, realization, surprise)
Boo-hoo! (crying, usually mocking since the crying sound can't be exactly represented in dialogue)
Sniff! (again, slightly weeping, expression of disappointment)
Woo-hoo! (Cheering, approval)
Surprise! (obvious interjection used to announce a surprise)
Whoa! (Stop or slow down)
Archaic or old-fashioned speech contains many interjections we still sometimes hear today, such as "alas" or "alack" (surprise), and "fiddlesticks" or "poppycock" (both mean nonsense or that something is ridiculous).
But keep in mind that many words or phrases taken out of the context of sentences could be classified as interjections. It is the usage of a word that often determines its part of speech.