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.
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.