branches and tags question

Hi all,

I have basic revision control question, which is basically, what is
the difference between a branch and a tag?

It is my understanding that fundamentally:

        - a tag is intended to never change, and is like a snapshot
          in time of the repository in a certain state.

        - a branch starts off similar to a tag, but evolves in a
          separate manner so that after the initial creation, there
          are differences between the branch repo and the trunk repo.

Is that correct? If so, how do I create a 'tag', but then insure
that it never changes? I understand that I can:

        svn cp http://path/to/repo/trunk http://path/to/repo/tag/tag-name

However, that does not prevent me, or anyone else, from accidently checking out
http://path/to/repo/tag/tag-name, making changes, and then
re-committing it.

Do I manually set the unix-level file permissions on the repository
structure to read only after the creation or is there an svn mechanism
to do this?

Thanks,

-- 
Seeya,
Paul
--
	It may look like I'm just sitting here doing nothing,
   but I'm really actively waiting for all my problems to go away.
	 If you're not having fun, you're not doing it right!
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