let me add some more
backup log will truncate the log
that means it will reuse the inactive portion of the transaction (few exceptions) log so it will prevent them growing
However it will never return back the space of the inactive part of the log file to OS when you do a backup.So shrink is the only option...
Changing the recovery model to simple and bring to full will help
and its better than truncate_only as it has been deprecated in sql 2008..so better stop using them