Why is this important? A good approach to creating CSS files is that you should put all of your common properties that will be the same for all media types in the 'all' media CSS file, and anything specific to displaying on a monitor in the 'screen' type, and anything for print preview or printing in the 'print' media type. This allows your display to be modified based on what device you are viewing the content on. This also helps to streamline your CSS files, as you keep out properties that are not needed or ignored for the device you are on.
There are several other media types that you could use, but the use of them is beyond the scope of this introduction. Some of them are: handheld, braille, aural, tv, and projection. These are not covered because their use is somewhat rare because many browsers do not implement them currently.
What you will want to do is determine what devices you will be supporting (often just screen and printer), and fashion specific CSS files for those devices. What you will not want to do however, is create full CSS files for each. That is, you will want to include only properties that are pertinent to that device type in your CSS file for that device type. This is called cascading, and is discussed more in-depth in the Cascading Section.
This might sound like an odd topic when discussing CSS, but it is a valid one. Graceful degrading, also sometimes called 'failing gracefully', is setting up your CSS files and importing them in such a way that older browsers and non-standard browsers which do not understand or only patially understand CSS do not load the CSS files at all. This produces an unstylized version of the page, which works in virtually every browser on every device, and is especially nice for small display devices such as cellphones and PDAs.
Below is a page that will degrade gracefully. In order to see this, you must load the below link with Netscape 4.x, IE 3, Lynx, or load it in Opera v7.21+ and hit Shift-F11. Otherwise you can load it on a PDA or cellphone, or an emulator as well. If you load the page with any current browser, you will see the normal version.