Interjections are sounds, words or phrases that express the speaker's emotion.
Here are some examples:
Ouch! It hurts!
Hmm... That's an interesting question.
Hey! What are you doing?
Wow, that's wonderful!
My goodness, what happened?
What is special about interjections is that they remain unaffected by the rest of the sentence. They do not change their form. They do not have singular/plural form, different tenses etc. Also, a single interjection can be considered as an entire sentence!
(That's the whole sentence...)
Here are some more examples:
You can use them to express emotions such as surprise (e.g. Oh!), annoyance (e.g. Damn!), joy (e.g. Hurray!), etc.
They are also used to greet (e.g. Hi!), pause your speech (e.g. Um...) and so forth.
When combining the interjection with the rest of the sentence you should usually use an exclamation mark (!) or a comma (,) depending on the strength of the statement.
Wow, this is awesome!
Ah, that's nice.