A Collection is a list which has child entities related to it together with metadata to describe itself. Collections are often used to group Assets and other Collections together as a way to organie them similar to creating a directory on a computer hard drive. Unlike a directory, members of a collection can be members of multiple collections at once.
There is metadata on the collection such as it's title and it's creator.
Additionally a collection can have an ACL applied to it that any member of the collection will inherit.