Starting with Mac OS X Lion (10.7) Apple decided to hide the Library folder within the User folder. It’s not gone, just hidden. Some crucial files like Mail accounts and iPhone backups get stored there, so there is a handy way to find the folder again.
To find the hidden library folder, go to Finder and click the Go menu at the top of the screen. You will see a list of folders. Now if you press the alt / option key on your keyboard you will see the hidden library folder appear. This is only a temporary way in, and you will have to press the key again next time. The folder remains hidden the rest of the time.