the "Metacourse" option is used to automatically enrol students in one course (metacourse or "parent course") if they belong to other ("child course"). If you enable this option and the course being exported is a parent or a child course in your site, that relations will be exported and the restore process will try to rebuild them in the destination server.
All users will backup all the users in your site, course will backup only the users needed (with some info) in your course and none, say, none!
The "userdata" switch near every module will determine if information related to users will be exported or no. For example, in a quiz, all the questions and quizzes will be exported always (module data), but attempts, answers and grades (user related data) will be decided by such setting.
Finally, "Uses Files" will export, for now, user photos (as they are all the files related to one user currently). In the future, it's highly probable that this option control other files owned by the user, DMS....).
Hope it helps...