The biggest difference between Macintosh OS 9 and OS X is that OS X is based on a version of the UNIX operating system, FreeBSD. The presence of UNIX isn't normally visible to OS X users. However, UNIX does influence many aspects of OS X indirectly.
User accounts
One concept that is foreign to pre-OS 9 users is individual user accounts. Prior to OS 9, only one user existed, and all files belonged to that user. OS X can also have multiple users, but the feature is organized the way UNIX does it. All users have their own home directory, and all their files are stored in that directory. All the home directories are found in a directory called User.
Users normally own their files. Each file in OS X has a set of permissions. These control who can read the file, write to the file, or execute the file as a program — another concept that comes from UNIX.
OS X knows which user you're by asking you to log on when you boot the machine. If you're the only user on the system, you don't have to log on, but from a security standpoint, it's best to require a logon.
File system structure
One big difference between OS 9 and OS X is how files are organized. In OS 9, your personal organizing touches start at your hard drive. In OS X, your playpen is your User directory. Outside of installing system-wide applications, you normally don't mess with files at the top level. Although you can see other user's folders, you normally can't poke around within them. Users get a certain amount of privacy in OS X, though if you have admin privileges, you can override most measures.
The OS X file structure seems awfully restrictive compared to the total freedom you're used to in OS 9. Some of the structure is arbitrary and goes back to the early days of UNIX. But the UNIX file system structuring has logic, and the ability to have separate user accounts requires something along these lines.
Interface differences
Apple made some small but annoying changes to the classic Mac OS interface when it created OS X. I outline some of these here:
The Apple menu is still on the far left of the menu bar, but its contents are almost totally different.
You can no longer easily add items to the Apple menu.
The cute little mini-apps that used to live on the Apple menu have been moved to a more elaborate Dashboard (press F12).
An application-specific menu now exists between the Apple menu and the File menu. It's where the Preferences, Hide, and Quit commands are located. This may be the hardest thing to get used to when switching to OS X.
The good news is that Command+Q still works, as do most of the command key shortcuts you're used to. One big exception is the shortcut that makes an alias for a selected file or folder, which is now Command+L. Also, Command+N now creates a new Finder window rather than a new folder. Finder windows are very handy in OS X, so this change makes some sense. Shift+Command+N makes a new folder.
Other popular OS 9 features are missing, including the Window Shade feature and tab folders. They've been replaced by the less-handy Dock. Fortunately, the Dock stack feature replaces tab folders, and a shareware program called WindowShade X restores the Window Shade trick.
Some things OS 9 users won't miss include system extensions and rebuilding the desktop. Neither are needed in OS X.
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