SharePoint 2007 now supports different "templates" for a document library. These are called "content types". Each content type can have it's own set of meta-data that you can apply to documents.
So how does this work when you try to save a document from MS Word 2003 up to SharePoint? Quite well actually. When you get to the screen that asks you for your meta-data, you will see a drop down menu of the different content types available in that library.
As you choose different content types, you will see the set of meta-data fields changing. Here is a screenshot of what it looks like: