Tag = a defined slice in time, usually used for releases
I think this is what one typically means by "tag". But in Subversion:
They don't really have any formal meaning. A folder is a folder to SVN.
which I find rather confusing: a revision control system that knows nothing about branches or tags. From an implementation point of view, I think the Subversion way of creating "copies" is very clever, but me having to know about it is what I'd call a leaky abstraction.
Or perhaps I've just been using CVS far too long.