Noun refers to the naming words which name the place, person, thing or animal. For example: I live in Delhi. Herein 'Delhi' is the noun.
Noun are categorized into various types. In the above sentence ' Everyone in the crew has completed many months of training.' Collective noun is 'crew'
Collective noun refers to the kind of noun which refer to the name of people, thing or anything in collective. For instance bundle of sticks, herein 'bundle' is collective noun. Therefore, in the above sentence 'Crew' refers to the the group of people on boat, ship, train and so on who work collectively.
