I don't seem to be able to find basic documentation of xwiki concepts, such as 'space', 'object'. Both of those words mean something to me, but it's not clear what they mean in the xwiki world.  Does xwiki presuppose familiarity with some other body of documentation? Where can I get this reference material?

We don't have any glossary yet. Feel free to start one in http://dev.xwiki.org/xwiki/bin/view/Drafts/ if you want.

However we do have descriptions of the terms you mention;

* Check http://platform.xwiki.org/xwiki/bin/view/Features/