Site Columns are the “properties” of a particular type of content. For example, the Site Columns of a Document Content Type might include Name, Description, Author, Status (such as Draft or Final), or Region. Site Columns can also be defined across the entire portal or for an individual site or site collection. This is one of the most exciting features of MOSS 2007, because “global” metadata management was not available in SharePoint 2003.
Essential SharePoint 2007