Library - Directly on HD vs. in User folder

Two questions:
1) What is stored in the library folder directly on the HD vs. the library folder in each of the User's folder?
2) Is the one directly on the HD important to include in a backup?

Thanks

The difference between equivalent folders in your home directory and the root directory (the top level of your hard drive) is the scope. The stuff in your home folder is only accessible to your user whereas the content in the root folder is available system wide.

The second question is difficult to answer without knowing how you're backing up content and what you hope of it. If you're doing full system backups, it's probably important. If you're just looking to ensure that the content you created is safe, probably not. A simple suggestion though - if you have the space, back it up.

There are actually three Library folders: /Library, users/user name/Library, and /System/Library.

The user name Library contains all your application support settings, keychains, other personal data, etc. (This folder is normally hidden by the system.)

It's recommended to backup everything. As long as you do not exclude anything from Time Machine, it will back everything up.