Microsoft's entry to the enterprise-level database market, competing against Oracle, IBM, and Sybase. The first version was SQL Server for OS/2 (about 1989) which was essentially the same as Sybase SQL Server 4.0 on Unix, VMS, etc. About the time Windows/NT 4.0 was coming out Sybase and Microsoft parted ways and pursued their own design and marketting scheme. Later, Sybase changed the name of its product to Adaptive Server Enterprise to avoid confusion with Microsoft SQL Server.
SQL Server uses a variety of SQL called T-SQL, a superset of SQL-92 (ISO standard for SQL, certified in 1992). T-SQL mainly adds support for transactions.
As of this writing (September 2001), SQL Server has ~14% of the commercial database market, according to Gartner Group.