This is from http://svnbook.red-bean.com/en/1.5/svn.branchmerge.tags.html
But wait a moment: isn’t this tag creation procedure the same
procedure we used to create a branch? Yes, in fact, it is. In
Subversion, there’s no difference between a tag and a branch. Both are
just ordinary directories that are created by copying. Just as with
branches, the only reason a copied directory is a “tag” is because
humans have decided to treat it that way: as long as nobody ever
commits to the directory, it forever remains a snapshot. If people
start committing to it, it becomes a branch.
Technically, it’s the same as a branch but conceptually we take it as a snapshot. In my svn repositories, I know that branches will include large features that may or may not have been merged into the trunk, but I use tags to mark release versions only.