In SVN there really is no technical difference between a tag and a branch IIRC, although their intended usages are different, as you say.
To the point where it's almost easiest to say SVN doesn't have branches or tags - it just has directories and conventions.
Git doesn't really have a technical difference between a tag and a branch either. They are both files containing a hash for some commit. Tags are just more cumbersome to move (branches are moved on new commits), and they can be signed cryptologically(is that a word?).
Except Git tracks history of the named branch in the reflog, and SVN (and others, I would guess) track it inside the repository. This is a huge difference for archeologists, but quite minor in practice from my experience.