Interjection comes from from a Latin word that means “throw between.” It’s a word or phrase that is thrown into a sentence to express an emotion:
Goodness, how you’ve grown!
Darn, I forgot my lunch!
Alas, will he never return?
All the impolite expressions that we call expletives are interjections. According to Wikipedia:
In linguistics, an interjection is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction.
Strictly speaking, an interjection is not a part of speech. It serves no grammatical function but is rather “a noisy utterance like the cry of an animal” (F.J. Rahtz). Interjections express feeling or emotion, not thought and have been called “the miserable refuge of the speechless.”
If you’ve ever stood lunch duty on a high school campus, you know just how vapid conversation can be when larded with meaningless interjections.
Here are some quotations from newspapers:
It was (like ours in theory but not, alas, in practice) a cosmopolitan age, when men everywhere recognized the great community of science and of learning; (www.nytimes.com)
Ack! $37,000 in college debt! Now what? Top tips from 2 authors – USA Today
You can get a comprehensive list of interjections used in the English language here: 100 Mostly Small But Expressive Interjections