Attended Installation — an interactive user interface will guide you through the set-up process and obtain all of the information and consent required to complete the installation. This includes the displaying, acknowledgement, and archiving of the applicable SQL Server license terms.
Un-attend Installation — this's a DOS command Line set-up process that does not have an interactive user interface to guide you through the set-up process. In this case, you will be required to enter a parameter during installation that indicates your acceptance of the license terms.
SQL Server 2008 R2 SP1 Express Edition Community Technology Preview Now Available!