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.
Portfolios that you submitted for assessment are not displayed per default unless you do so via the option "Show submitted portfolios".