SQL Server is a relational database server used in SharePoint

Microsoft SQL Server is a relational database server, developed by Microsoft: it is a software product whose primary function is to store and retrieve data as requested by other software applications, be it those on the same computer or those running on another computer across a network (including the Internet)

Questions with the SQL Server tag regards development, configuration and implementation of content and structure database in SharePoint.

Reference: http://en.wikipedia.org/wiki/Microsoft_SQL_Server