To administer an ArcSDE database server, you add a database server connection in the Catalog tree. This is a connection to a SQL Server Express instance set up to store geodatabases. Adding a database server creates a database server connection file and an icon in the Database Servers folder for that connection.
Once you are connected to a database server, you are able to administer the database server itself as well as add, remove, or administer the geodatabases on the database server.