A transitive verb transfers the action from the subject to the object.  For example:  Tom threw the ball.  The subject is Tom.  The verb is "threw" and it is transitive.  The reason why is because "ball", the object receives the action.  It is what is thrown.  The action is transferred from Tom to the ball- from subject to object- by the verb "threw". 

Here is the same verb as an intransitive verb:  Tom threw all day.  Here, Tom is still the subject and he still throws.  However, we do not know what or whom he threw.  We just know he threw whatever it was all day.  No action is passed- so therefore, the verb is intransitive.