The Library Folder in OS X Yosemite
The Library folder, at the root level of your OS X hard drive, is like a public library; it stores items available to everyone who logs into any account on this Mac.
There are actually three or more Library folders on your hard drive:
At the root level of your OS X disk
In the root-level System folder
In each user’s Home folder
Now, here’s the scoop on your various Library folders:
The “Public” Library: You find a bunch of folders inside the Library folder at root level (the “public” Library folder). Most of them contain files that you never need to open, move, or delete.
By and large, the public Library subfolder that gets the most use is the Fonts folder, which houses many of the fonts installed on the Mac.
System Library: This is the nerve center of your Mac. In other words, you should never have to touch this particular Library folder.
Leave the /System/Library folder alone. Don’t move, remove, or rename it, or do anything within it.
Library in each user’s Home folder: This is where OS X stores configuration and preferences files for each user account.
The locations of all these libraries are illustrated here.
If your Mac is set up for multiple users, only users with administrator (admin) privileges can put stuff in the public (root-level) Library folder.