A copy of the sqlservr.exe executable that functions as an operating system service is referred to as an instance of the database engine. A number of system databases and one or more user databases are managed by each instance. The Database Engine can run in numerous instances on each computer. Applications connect to the instance to work on databases that the instance manages.
Every application request to interact with the data in any of the databases that the instance manages is handled by an instance of the Database Engine running as a service. The connection requests (logins) from applications are directed at it.
You can run multiple instances of the Database Engine on a computer. One instance can be the default instance.
The default instance has no name. If a connection request specifies only the name of the computer, the connection is made to the default instance.
* The installed instances can be viewed in the registry editor.
A named instance is one where you specify an instance name when installing the instance. A connection request must specify both the computer name and instance name in order to connect to the instance.
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