There are a few reasons why you might want to stop CDs automatically playing when you insert them. If you don't use the standard Apple CD Audio Player for your music CDs (perhaps you use iTunes or one of the many third party programs) it gets annoying to have to stop the Apple Player (although you can use the Control Strip CD icon for this) then start it again from the software you really want to use.
With CD ROMs you might not want to have to wait for the interface on the CD to load up, for example if you're using one of the many magazine cover mount CDs that also let you get at the software through the Finder. This is a particular problem if you have an older machine that might take a few minutes to load the interface.
There's another good reason to disable AutoPlay, the Autostart 9905 Worm, which was the last major Mac specific virus that appeared back in 1998. This worm can only affect machines which have the CD AutoPlay feature turned on. Many Mac users don't have any virus protection, mainly because they don't think it's a worthwhile investment because of the small number of viri that can affect Macs. If you're in that category, turning AutoPlay off may be a wise move.
The actual On/Off switch for CD AutoPlay isn't in a particularly obvious place. It's in the QuickTime Settings control panel, which you'll find in the Control Panels section of the Apple Menu if you've got QuickTime 3 or later.

Use the pulldown menu at the top to select the AutoPlay section. Now it's simply a case of checking or unchecking the boxes to set your preference. The settings are active from the next time you insert a CD, you don't need to restart the Mac.
