SQL Server
SQL Server describes any Database Managment System which allows the management and manipulation of any SQL based database. Examples of a SQL Server system would be Microsoft SQL Server and MySQL, an open source SQL implementation.
SQL server implementations comprise a complex set of programming structures that allow the manipulation of any SQL based database system, so that not only can data be manipulated, but also structure be implemented. SQL Server implementations therefore abstract control from the older command line interfaces used to program and manipulate databases.
For a full and complete description of SQL server architecture, see HERE