Essentially none. There are three Library folders: Macintosh HD/System/Library/ Macintosh HD/Library/ Macintosh HD/Users/youruseraccount/Library/ The first belongs exclusively to the operating system and is accessible only to the root user; The second belongs to all users on the computer but is only accessible to the admin and root users; The third belongs to the specific user. It is accessible to the specific user and the root user. Generally, you should never attempt to access the first Library unless you really know what you are doing and why you are doing it. Under most conditions you should have no reason to access the second level Library. You can access the Library folder in your user account, but there usually is no need to do so, and you should know what you are doing if you do access it.