Library Folder in OS X Yosemite contains preference files, caches, personalizations and other contents. Most of Mac users won’t need access to their Library Folder but some advanced users want to access to that folder. Through OS X 10.6 the Library folder is hidden to prevent erroneous access. With OS X Yosemite you can use a simple settings to reveal that folder.
Close the View Options panel and will see the Library Folder. If you had previously set to broadly show hidden files across OS X then Library directory would be visible in the users home folder.
If you open Library Folder, you will see many of contents specific to that user account, fonts, caches, scripts, application files and much more. Don’t modify any of these files or folder if you don’t know what you are doing.