From Apple Help:
About using Mac OS 9 applications in Mac OS X
If you need to use Mac OS 9 applications, you can do so in the Classic environment in Mac OS X. To use the Classic environment, you must have a Mac OS 9 System Folder installed on your computer, either on the same hard disk as Mac OS X, or on another disk or disk partition.
The Classic environment requires Mac OS 9.1 or later. If you are using an earlier version, you need to upgrade your Mac OS 9 software to use the Classic environment. It's a good idea to use Mac OS 9.2 or later.
For best results using Mac OS X, upgrade your Mac OS 9 applications to the Mac OS X version. Check with the manufacturers for more information. You can also check the Mac OS X website for information about the latest applications for Mac OS X.
If your computer didn't come with Mac OS 9 installed, use the discs that came with your computer to install Mac OS 9.
To intall Mac OS 9:
Back up your essential files, if possible.
Insert the "Additional Software & Apple Hardware Test" disc.
Double-click "Install Extra Software."
Follow the onscreen instructions to install Mac OS 9.
Your computer may have come with multiple CDs, or discs with different titles. See the documentation that came with your computer to determine which discs to use.
The Classic environment requires at least Mac OS 9.1, but it's a good idea to use Mac OS 9.2 or later. You can download a software update from the Apple support website.
To learn more about using Mac OS 9 applications in Mac OS X, click "Tell me more."