Microsoft SQL Server is a relational database from Microsoft. Its origins are in Sybase SQL Server, when Microsoft and Sybase jointly created a version of it for IBM OS/2. When Sybase and Microsoft parted ways, Microsoft reserved SQL Server name for all releases of the software on Microsoft operating systems. Somewhat ironically, Sybase had to change name of its own product later to Sybase Adaptive Server Enterprise to avoid confusion with Microsoft SQL Server.
While Microsoft SQL Server 7.0 and 2000 still extended and modified Sybase database server code, starting with SQL Server 2015, Microsoft has completely rewritten all of the code, so its product no longer contains any Sybase codebase.
Microsoft SQL Server is extremely popular database, with almost the same popularity as Oracle and MySQL. Oracle is still used for the largest installations way more often than SQL Server, being a distributed database right in its core.
SQL Server is an engine of choice for Microsoft .NET developers as it’s tightly integrated with Microsoft Visual Studio and allows for easy development. Starting with SQL Server 2005, SQL Server even hosts Microsoft .NET Framework runtime itself - so stored procedures, functions and triggers can be written in any .NET language (C# and VB.NET being most popular).
One of the Microsoft SQL Server’s strengths is certainly inclusion of valuable business technologies, such as Analysis Services (OLAP and data mining), Reporting services, Integration Services (ETL for data import, integration and data warehousing needs).