Spaces - this is the "uniting" tool for the content from the different apps but with the one "purpose". For example, you may create the space of some singer and then post there his images, videos about him etc. Also, it has the possibility to create a parent and child spaces (big Space "singers" and then sub-spaces like Sinatra, Elvis, Dylan etc).
Ok, i been playing with spaces and from what i can see, author creates a space and invites members. After author creates a space and sets privacy to secret, no one else can view what is discussed in the "space correct?"
Well, not sure about the biography. The nice example of the "Space" can be directories and fields - every directory may have the own sub-directories and the files there. So you may apply this structure as you want. Yes, you may setup your space as closed for non-spaced members.
One of the key advantages of Spaces is the capability of nesting Groups, within Groups - as Leonid describes above 'Singers' would be a Group, then 'Elvis' would be a Group - you could potentially expand that a have additional groups with 'Drummers' as a Group and Lars Ulrich as a Group within that group.
If I understood spaces right than spaces seems to be a great concept for nested content, as it is not "bound" on a specific content type but brings content together both hierarchical an horizontal. As spaces can have spaces, and spaces can be used as container, the possibilities seem to be endless....