Differences between startup, startup mount, and startup nomount

RMAN> startup nomount
Connected to the target database (not started)
Oracle routine started
The system's global region consists of 135338868 bytes.
Fixed size 453492 bytes
Variable Size 109051904 bytes
Database buffers: 25165824 bytes
Redo buffers: 667648 bytes

RMAN> startup Mount
Connected to the target database (not started)
Oracle routine started
Database loaded
The system's global region consists of 135338868 bytes.
Fixed size 453492 bytes
Variable Size 109051904 bytes
Database buffers: 25165824 bytes
Redo buffers: 667648 bytes

RMAN> startup
Connected to the target database (not started)
Oracle routine started
Database loaded
Database opened
The system's global region consists of 135338868 bytes.
Fixed size 453492 bytes
Variable Size 109051904 bytes
Database buffers: 25165824 bytes
Redo buffers: 667648 bytes

The above three types are available. startup nomount only starts the instance but does not start the database. startup Mount starts the instance and loads the database, but the database is not opened. startup is the most complete instance, the database is loaded and opened.

For example:

Microsoft Windows XP [version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:/Documents and Settings/Administrator> Cd/

C:/> sqlplus/nolog

SQL * Plus: Release 9.2.0.1.0-production on Monday September 21 15:21:50 2009

Copyright (c) 1982,200 2, Oracle Corporation. All rights reserved.

SQL> connect/@ dwj as sysdba
Connected.
SQL> startup nomount
ORA-01081: unable to start Oracle that is already running --- Close first
SQL> shutdown abort
The Oracle routine has been disabled.
SQL> startup nomount
The Oracle routine has been started.

Total system global area 135338868 bytes
Fixed size 453492 bytes
Variable Size 109051904 bytes
Database buffers 25165824 bytes
Redo buffers 667648 bytes
SQL>