Being a multi-user operating system, OS X sequesters settings and personal data in separate home folders, which are accessed when the respective user logs in. By default, the contents of each user's folder are off-limits to another user, which is good for security but can be a burden if one user would like to share files or folders with another.
For example, if you save a Word file to your Documents folder, or even to your Desktop, then another user will not be able to access it from his account. One approach here is to modify the default permissions on the Desktop or Documents folder; doing this will make all contents of the folder available to other users, which may compromise security and not be the desired solution.
If you have documents saved in your account that you would like other users to access, then there are several approaches you can take.
The first is to use your account's Public folder. Even though the default folders in your account, such as the Desktop, Document, and Movies, are all off-limits to other users, the Public folder in your account should be readable by everyone (with a "Drop Box" folder that is writable). Using this, you can store a file or two for others to copy or read, and accept files from them in the Drop Box.
While using the Public folder is one option, its read-only permissions (and write-only permissions for the Drop Box subdirectory) mean that if you use this setup regularly, you can end up with multiple copies of the file in each user's Public and Drop Box folders as they get copied back and forth, which may be a bit of a burden to manage.
The second option is to use the system's "Shared" account folder, which is located in the Macintosh HD > Users directory, and should be fully accessible by all users to read and write files. This option may be more appealing than the Public folders, especially if you are sharing files that are regularly accessed, such as a common iTunes library for all users.
Another option is to use secondary partitions, which include those on the main boot drive, secondary internal drives, and external drives. Similar to the Shared folder, these locations will be fully accessible to all users by default. The benefit of using these is that the Finder has preference for users to display them on their desktops and in the Finder sidebar, so they will be quickly accessible by everyone with a simple Finder settings change instead of having to navigate to a specific folder path.
The aforementioned approaches all require intermediary folders or storage options, and are great if you plan on moving files back and forth between accounts; if you only wish to move files to one account, then you can use a standard authenticated move feature of the OS X Finder. To do this, simply select the files you wish to move, and drag them to the Documents, Movies, Music, Desktop, or other restricted folder of the targeted account. Since you cannot open the folder, you can only access the top level of the folder tree in this manner (i.e., the Desktop folder itself, and not a subdirectory in it). When you do this, the system should issue you a warning about not being able to access the folder, but provide an option to authenticate and continue moving the files.