In Subversion terminology this is called 'tagging' the code.
From the root of a local subversion workarea do this:
svn copy -m "Tag for demo released on June 26, 2009" . http://my.svn.host/path/to/project/tags/demo-090626
this will create a tag called demo-090626 by copying the current workarea to the tag.
You will have to change a couple of things:
* The server may be running under SVN protocol instead of HTTP (change http:// to svn://)
* Fix the subversion server hostname in the URL.
* Change /path/to/project appropriate for the path on your SVN server (use 'svn info' and look at the URL field in the output).
* You might have to create the tags directory on the SVN server for your project if it doesn't exist yet (do 'svn help mkdir'). Important: the tags directory must be a sibling to the trunk/ directory, not a child directory.
Hope this helps,