Site Column is one of the new features in the new version of SharePoint, MOSS 2007.
A Site Column (SC) is a reusable column definition that can be used in multiple lists across multiple SharePoint sites. This will be required when the organization need to have a consistent site design or components across the lists and libraries. SCs are attached to native column types which are made available in WSS 3.0
For instance, if your organization had a vendor site collection build on MOSS, then all the sub sites would have a common skeleton (data structures in libraries and lists) to capture the data. Defining a site column at the top level will help you to reuse the same columns across mutiple sites within the site collection. Eg:- RFQ (Request For Quote) document library in the vendor subsite in the above site collection . You can defined the columns required to capture the RFQ data and make use of those columns in all subsequent sites.
Again, after creating the site column in the top level site and leveraging it in the site columns in sub sites, any change in the definition of the site column can be reflected in all the sub sites. That is, if you make changes to a site column, you can update all the list columns based on that template. This is done by pushing down the changes you made to the site column. Pushing down is not an all-or-nothing operation: if the push down of changes fails on one list, Windows SharePoint Services continues pushing down the changes to the remaining lists. Windows SharePoint Services returns a list of errors encountered at the end of the push-down operation.
However, deletion of site column will not delete the columns from the lists where it has beeen used.
In other words, SC can be compared to the User Defined Data Types (UDT) in sql server. In SQL, the UDTs are built on native data types available in the server, say int, money, varchar, etc. Similiar to that, SC is built on native column types available in Windows SharePoint Services 3.0.