well "hooks" are basically programs that get triggered when a particular svn event happens. Accordingly, the post-commit hook gets triggered after every commit. If you have access to your svn repository (as an admin, not just as a user), you should go to /path/to/repo/hooks
directory and see the templates of various hooks that get run. Use the post-commit.tmpl file there as a template and add whatever you want. Note that whatever program/script gets run as the hook, automatcially gets arguments like name of the repo, current revision being checked in etc. as command line arguments. See here for reference:
http://svnbook.red-bean.com/nightly/en/svn.reposadmin.create.html#svn.reposadmin.create.hooks
Then you can take the arguments and do whatever you like after every commit, including counting diff lines or what have you.