What are the recommended file and directory permissions?
From Joomla! Documentation
This article describes filesystem permissions under UNIX-type operating systems.
Permissions are a common source of trouble in Windows as well, especially under the IIS webserver. Find that information at IIS And Joomla.
UNIX/Linux[edit]
Depending on the security configuration of your Web server the recommended default permissions of 755 for directories and 644 for files should be reasonably secure.