The OS X Mountain Lion Users Folder
When you open the Users folder in OS X Mountain Lion, you see a folder for each person who has a user account on the Mac, as well as the Shared folder.
The Shared folder that you see in the Users folder allows everyone who uses the Mac to use the files stored there. If you want other people who use your Mac to have access to a file or folder, the Shared folder is the proper place to stash it.
A lot of people don’t share their Macs with others. OS X is based on the Unix operating system — a multiuser operating system used on high-end servers and workstations that are often shared by several people.
OS X has both the benefit of this arrangement and a bit of the confusion caused when a single user (could it be you?) fires up a computer that could be set up for several people. That’s why OS X folders are organized the way they are — with different hierarchies for each user and for the computer as a whole.
Each user account has its own preferences and settings, and that one user can’t see another user’s files by default.
If your Mac has more than one constant user, consider creating a user account for each person. If you’re a parent, separate user accounts mean you can enable Mountain Lion’s terrific Parental Controls, which are a very good thing.