English grammar
Interjections are words that do not fit normal grammar rules.  Interjections can and often do take the place of an entire sentence, as they can give they meaning of a whole sentence in a single word.  These can be used to show emotions, such as the word "Hooray," which means that the speaker is happy or likes something.  They are also used to shorten common phrases that would otherwise need a full sentence to talk about. For example, saying the word "yes" is much simpler than saying "what you say is true," so it is usually used instead.  Interjections like these can be helpful for saving time and making complex sentences very simple.  Often, though, interjections may have no meaning at all, such as the word "um."