Interjections express a strong feeling or sudden emotion.
They usually bring a brief and abrupt pause in conversation when the emotion is expressed.
They can express emotions like surprise, joy, disgust, relief etc. and may include exclamations, swearing, greetings, or just sounds.
They may contain just one word or be a non-sentence phrase.
They are often written with an exclamation mark at the end.
Wow! Oops Ewww Hurrah! Hello! Boo! Phew! Goal!! Shh! Pfff!
Argh Psst Ohh Thanks! Uh-huh Hey! Nah Shoo Bravo! A-ha
Wow! That’s a crazy price for a handbag!
Ewww! There’s a hair in my sandwich!
Hey! Pass the ball over here.
There are hundreds, possibly thousands of interjections. For an idea of just how many see Wiktionary