Library folders contain information used by the Mac OS and applications. There are probably several Library folders on your computer.
The Library folder at the top level of your hard disk contains files that are important for the Mac OS and that apply to all users of the computer. For example, the Fonts folder contains fonts available to all users.
Each user account also has its own Library folder, which contains files used just by that account by the Mac OS and applications. Fonts in a user's Library folder, for example, are available only to that user.
Note: This information comes from Mac Help, the help system installed with Mac OS X 10.3 Panther. To access Mac Help on your computer, choose Mac Help from the Finder's Help menu.