For Oracle, we mount the instance to the database or mount the database to the instance? The following text is extract from oracle documentation.


  Start the instance, and mount and open
  the database. This can be done in
  unrestricted mode, allowing access to
  all users, or in restricted mode,
  allowing access for database
  administrators only.
  
  ....
  
  To mount a database to a previously
  started, but not opened instance, use
  the SQL statement ALTER DATABASE with
  the MOUNT clause as follows:
  
  ALTER DATABASE MOUNT;


You can read it directly from http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/start.htm

The first part gives me an impression that we are mounting the instance to the database. But the To mount a database to a previously started, but not opened instance part gives me an impression that we are mounting database to an instance. 

Which one understanding is right? Anyone can help to explain it please?

Thanks
We are mounting a database to an instance. See the concepts guide:


  The instance mounts a database to
  associate the database with that
  instance.


One way I've seen it explained (probably here or on SO) is that the instance is the memory structures and processes, and the database is the data on disk. The instance has to have a database associated with it to be able to do anything useful.