When you buy a normal itunes file it is protected with DRM, a digitial copyright management system. This means the fiel can only be on 5 seperate computers as you need to authenticate ourself to the system before you can play it. You can only play this music on an IPOD.
Itunes Plus, is free of such nonsense! download it, put it on your mp3 player, and put it on as many puters as you need without having to authenticate.
Also, iTunes Plus is higher quality. This may be noticeable if you plan to listen to it through your hifi, and if it's something like Pink Floyd's Dark Side of the Moon.