Wow~~beside VS 2005 Express Family, there is also a new Edition for SQL Server 2005, which is also called Express Edition. It is an entry-level database for hobbyists, enthusiasts, and students developers and so the same “EXPRESS“ to VS 2005. The SQL Server Express is a version of SQL Server 2005 designed for building simple, dynamic applications. SQL Server Express helps developers build robust and reliable applications by providing a powerful database that is also free and easy to use. Too often database systems are overly complex for building simple applications. SQL Server Express and the other Express products reduce this complexity by providing a simple but powerful development environment for building data-driven applications. You can design schemas, add data, and query local databases, all inside the Visual Studio 2005 environment. If you need more advanced database features, then SQL Server Express can be seamlessly upgraded to more sophisticated versions of SQL Server.
More info? Here you are,
http://lab.msdn.microsoft.com/express/sql/