First of all, what does archiving a project mean to the end users? Obviously it stops users from connecting to the project. What else? Where is the archive? Is there a file created after the archiving action?

Archiving a project area is considered a soft-delete. The project area can no longer be accessed by end users but it's still accessible for audit purposes (see Archived Project Areas section in Jazz Project Administration - http://publib.boulder.ibm.com/infocenter/rqmhelp/v2r0/topic/com.ibm.rational.test.qm.doc/topics/t_create_project.html). Note, the archive operation cannot be undone, so be sure that you want to archive the project area.

Secondly, is there a way to export the project and its children including all test artefacts of that project to an external file, e.g. .zip And then later, this file can be re-imported to any RQM to be an active project again?

You can export the complete database but not specific project areas. See:

http://publib.boulder.ibm.com/infocenter/rqmhelp/v2r0/topic/com.ibm.jazz.install.doc/topics/c_repotools_overview.html