Permissions are used to grant different access levels to your files and folders. You can share them as Read Only, or you can give users Read & Write access, or simply deny other users access to your files and folders.
Mac OS X provides a common folder for all local users at /Users/Shared.
Folder permissions and file permissions can be combined to limit access in very sophisticated ways.
References
The following Knowledge Base documents (located at www.apple.com/support) will provide you with further information regarding users and permissions in Mac OS X.
Files
106237, "Unable to move, unlock, modify, or copy an item in Mac OS X"
Permissions
25751, "About Disk Utility's Repair Disk Permissions feature"
25554, "Mac OS X: File can't be moved if locked"
106712, "Troubleshooting permissions issues in Mac OS X"
107031, "Mac OS X 10.2, 10.3: Clicking 'Apply to enclosed items' copies permissions but not owner or group"
107039, "Mac OS X 10.2: How to Change Ownership & Permissions Using the Finder"
Trash
106272, "You can't empty the Trash or move a file to the Trash in Mac OS X"