A page contains a selection of artefacts which you arrange and present to others. These may include:
You can re-use the artefacts that you collected under "Content" in as many pages as you wish.
A collection is a set of pages that are linked to one another and have the same access permissions. You can create as many collections as you like, but a page cannot appear in more than one collection.
You can create a collection from scratch or copy an existing one - either one of your own or one from somebody else that is copyable to you.