From Wikipedia..."Microsoft SQL Server is a relational database management system developed by Microsoft. As a database server, it is a software product with the primary function of storing and retrieving data as requested by other software applications which may run either on the same computer or on another computer across a network (including the Internet)."

From Ross...Microsoft SQL Server is an excellent mainstream database engine that can work with any datatypes and provides all modern database functionality. It can be managed and used through the MS SQL Server Management Studio or the Visual Studio plugins. I prefer to use SQL Server whenever doing new development that does not already have it s' database on some other system. Connection to a SQL Server db in a .net application is quick and headache free. It's also very convenient to access the management tools for the db from the same instance of Visual Studio in which you are developing the application code.