When we go to the assets we got two options which is file and collection. What is the difference between these two. Please let me know.
As per documentation 
A collection is a set of assets within Adobe Experience Manager (AEM) Assets. Use collections to share assets between users.
A file 
Is the real asset that could be used managed into the DAM.
 Managing Collections
 Administering Assets
Please explain in basic difference.
in really basic word, a collection is a kind of container instead of a file that is the elements that you can put into a collection.
It's similar to a folder and a file into a folder in an operative system.
Hope this help.
so, when we use collection and when we use files?