SQL Server Service
The Microsoft® SQL Server™ 2000 database engine runs as a service on the Microsoft Windows NT® or Microsoft Windows® 2000 operating systems. It does not run as a service on Microsoft Windows Me or Windows 98 because these operating systems do not support services. SQL Server can also run as an executable file on Windows NT and Windows 2000, although it is usually run as a service.
When multiple instances of SQL Server are run on the same computer, each instance has its own SQL Server service. The service name for the default instance is named MSSQLServer, the service name for named instances is MSSQL$InstanceName. For more information, see Multiple Instances of SQL Server.
The SQL Server service manages all of the files that comprise the databases owned by an instance of SQL Server. It is the component that processes all Transact-SQL statements sent from SQL Server client applications. SQL Server also supports distributed queries that retrieve data from multiple sources, not only SQL Server.
The SQL Server service allocates computer resources effectively between multiple concurrent users. It also enforces business rules defined in stored procedures and triggers, ensures the consistency of the data, and prevents logical problems such as having two people trying to update the same data at the same time.