Site columns allow you to create re-usable definitions for columns that can be added to any list across the site collection. In this video, Hilton Giesenow, host of The MOSS Show SharePoint podcast ( http://www.TheMossShow.com/ ) shows how to use Visual Studio 2010 to create your own site column for use in your SharePoint lists and libraries as well as how to use the new validation feature of Site Columns in SharePoint 2010.
More here:
How Do I: Create a Site Column in Visual Studio 2010?