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 organize 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.
Collection content is unordered by default, and returned via the search engine in an ordered determined by the search query, however if you have chosen to order the collection this ordering can also be used when returning the results.
Additionally a collection can have an ACL applied to it that any member of the collection will inherit.