The System section includes preferences that control a number of software features on your Mac, including Accounts, Classic, Date & Time, Software Update, Speech, Startup Disk, and Universal Access.
Classic is the Mac OS 9 operating system you’ll need for the next year or two until all of the software you use has been updated for OS X. When you open an OS 9 application, it will automatically open in Classic. Essentially, you will be switching between two different operating systems. If you have no OS 9 applications, you will never need to use Classic and you don’t even need to install it.
If you do use Classic, I recommend ...