ALTER SYSTEM

Modify system settings.

Syntax:

   ALTER SYSTEM ARCHIVE LOG archive_log_clause
   ALTER SYSTEM CHECKPOINT [GLOBAL | LOCAL]
   ALTER SYSTEM CHECK DATAFILES [GLOBAL | LOCAL]
   ALTER SYSTEM FLUSH SHARED POOL
   ALTER SYSTEM {ENABLE | DISABLE} DISTRIBUTED RECOVERY
   ALTER SYSTEM {ENABLE | DISABLE} RESTRICTED SESSION
   ALTER SYSTEM RESUME
   ALTER SYSTEM SUSPEND
   ALTER SYSTEM SHUTDOWN [IMMEDIATE] dispatcher_name
   ALTER SYSTEM SWITCH LOGFILE
   ALTER SYSTEM KILL SESSION 'int1, int2' [POST TRANSACTION] [IMMEDIATE]
   ALTER SYSTEM DISCONNECT SESSION 'int1, int2' [IMMEDIATE]
   ALTER SYSTEM SET parameter = value [COMMENT 'text'] [DEFERRED] [Scope_options]
   ALTER SYSTEM RESET parameter(s) [scope_options]
   ALTER SYSTEM QUIESCE RESTRICTED
   ALTER SYSTEM UNQUIESCE

SET Parameters:
   Full list of Static and dynamic parameters.

   Scope_options:
   SCOPE = {MEMORY|SPFILE|BOTH} [SID = 'sid' [,SID =...]]

   DEFERRED - Set parameter values for future connecting sessions.
              Currently active sessions are not affected and they retain the old parameter value.
              This is required for parameters that have the ISSSYS_MODIFIABLE =DEFERRED (in V$PARAMETER) 
              This is optional for parameters that have the ISSSYS_MODIFIABLE =IMMEDIATE (in V$PARAMETER)
              For static parameters, DEFERRED cannot be specified.

   COMMENT text is visible in the UPDATE_COMMENT column of V$SpPARAMETER

   The SID clause is for RAC, where it's possible to change a parameter value for one instance only.
   SID= '*' will affect all instances on the cluster
   If the instance is started with an SpFILE the default = * (all instances)
   If the instance is started with a PFILE then default SID = the current instance.

Quiesce

Putting the database into a quiesced state will prevent inactive user sessions from becoming active, Oracle then waits for existing transactions to finish. In shared server mode, Oracle will also block user logins.
This allows SYS/SYSTEM to perform some maintenance tasks without a full shutdown.

Notes
All Byte values can also be specified in K or M or G

e.g. you can enter 8388608 or 8192 K or 8M

All directory paths follow standard notation i.e UNIX 'quotes' or Windows "double quotes"

"An expert is someone who knows some of the worst mistakes that can be made in his subject and how to avoid them." - Werner Heisenberg

Related:

ALTER DATABASE
ALTER SESSION
SET TRANSACTION
STARTUP
Full list of Static & Dynamic Parameters - init.ora
Server Parameters - OraFaq.com

Related Views:

 DBA_DATA_FILES
 V$ARCHIVE
 V$ARCHIVED_LOG
 V$ARCHIVE_DEST
 V$ARCHIVE_PROCESSES
 V$PARAMETER
 V$SpPARAMETER
 V$SHARED_POOL_RESERVED 
 V$SESSION