If they are on the same server and same database instance then you can just reference the database by using … [DatabaseName].[DatabaseOwner].[TableName]
If it is on another server then you will need to link the servers.
To the best of my knowledge, and I am not a SQL expert, I think SQL Server may treat different SQL instances as different Servers even if they are on the same physical server, that should be in the help files.
Hope this helps.