Library Subfolder of Your Mountain Lion Home Folder
The invisible Library subfolder of your Mountain Lion Home folder is the repository of everything that OS X needs to customize your Mac to your tastes. If you want to add something to a Library folder, it’s usually best to add it to your Home/Library folder.
You won’t spend much time (if any) adding things to the Library folder or moving them around within it, and that’s probably why it’s now hidden from sight.
Be cautious with all Library folders. OS X is very persnickety about how the folders and files within it are organized. You can add items to and remove items safely from most Public or Home Library folders, but leave the folders themselves alone. If you remove or rename the wrong folder, you could render OS X inoperable.
It’s like the old joke about the guy who said to the doctor, It hurts when I do that, and the doctor replies, Then don’t do that.
To find your hidden Home/Library folder, do this:
Hold down the Option key on your keyboard.
Click the Go menu.
The (formerly) invisible Library folder appears in the Go menu as long as the Option key is pressed.
Select Library and release the mouse button.
You should see several folders in the Home/Library folder; the exact number depends on the software that you install on your Mac. You probably have folders called Mail, Safari, Logs, and Preferences, for example.
Some of the most important standard folders in the Library folder include the following:
Application Support: Some applications store their support files here; others store theirs in the main (root-level) public Library folder.
Fonts: This folder is empty until you install your own fonts here. The fonts that come with OS X aren’t stored here, but in the Library folder at root level for everyone who uses this Mac. If you want to install fonts so that only you have access to them, put them in the Fonts folder in your Library folder.
The easiest way to install a font is to double-click its icon and let Mountain Lion’s Font Book utility handle it for you. Here's how to install a font manually:
To install a font that only you can use: Drag the font file’s icon to the Fonts folder in your Home/Library. The font is available only to this user account (because other users can’t use fonts stored in your Home/Library folder).
To install a font for all users of this Mac: Drag the font file’s icon into the Fonts folder in the public Library folder — the one at root level that you see when you open your hard drive’s icon.
Preferences: The files here hold the information about whichever things you customize in OS X or in the applications you run. Whenever you change a system or application preference, that info is saved to a file in the Preferences folder.
Don’t mess with the Preferences folder! You should never need to open or use this folder unless something bad happens — if, for example, you suspect that a particular preferences file has become corrupted (that is, damaged).
If you don’t know why you’re doing something to a folder (other than the Fonts folder) in your Home/Library, don’t do it.