Interjection is related to an expression of feeling, rather than representing some idea or concept. 

Generally, interjections can be classified into 

three types of meaning: volitive, emotive, or cognitive

Volitive interjections function as imperative or directive expressions; requesting or demanding something from the addressee (e.g., Shh! = "Be quiet!" Boo! as in "Boo!" she cried, jumping to frighten him).

Emotive interjections are used to express emotions, such as disgust and fear (e.g., Yuck! expressing disgust Boo! signalling contempt as in Boo! Shame on you or by audience members or spectators after a performance).

Cognitive interjections express thoughts which are more related to cognition, or information known to 

the speaker of the utterance (e.g., Um! indicating confusion or thinking )