what is differerence between common and collective noun?

A common noun is a word that does not specify a particular person, place or thing. It, rather, represents a class or a kind. For example:

Books are man’s best friends.

Rachel is an honest girl. 

A collective noun is a word that talks about the people and things spoken of as a group and not as single. For example:

The Indian army fought bravely. 

The committee decided to organise an inter-school tournament.