Another type of data access, which was
added to the product in SQL Server 2005, is access through the version store.
Row versioning allows SQL Server to
maintain older versions of changed rows. The row versioning technology
in SQL Server supports Snapshot isolation as well as other features of SQL
Server 2008, including online index builds and triggers, and it is the
versioning operations code that maintains row versions for whatever purpose
they are needed.
See my another article for more detail
