What is an intransitive verb?

intransitive verb has no object to receive action of verb e.g., i am singing a song, in this action of singing is transferred to song, so singing is transitive,

but i am singing , in this action of singing stays , not transferred, so its intransitive 

other examples: he closed the door (transitive) why? he --> close --> door (action moves from subject to object) ask he closed what? door!

she came home late (intransitive) why? she ---> came ---> (no object) ask she came what? no answer!

That is why, sentence containing transitive verb has active voice and passive voice

The door was closed by him (passive)

A song is being sung by me (passive)

sentence containing intransitive verb has only active voice