Q: What is the Library folder in Mac OS X used for?

Answer

The Library folder is a portion of the operating system. Every user directory has its own Library folder. Each user can customize applications and the operating system to their liking and not interfere with other user's customizations.

There are actually three locations on the hard drive where Library folders exist. Each location governs a different part of the operating system.

/System/Library is the location for the core parts of the operating system needed by your Mac to function.

/Library is the location for the parts of the operating system that apply to ALL users and applications running on the Mac like fonts and system wide application preferences.

/Users/<user home directory>/Library is the location for all the user specific parts of the operating system like preferences, favorites, recent items, etc.

**NOTE: In macOS 10.8 and higher, the /Users/<user home directory>/Library is a hidden folder. This means you will not not see it by default. 

If you need to access the /Users/<user home directory>/Library, please visit this link.