A user process is created and maintained to execute the
software code of an application program. It is a shadow
process created automatically to facilitate communication
between the user and the server process

A configuration for loosely coupled systems where multiple
instance share a single physical database is called Parallel
Server. 

it writes the uncommited and the comited data from the
redolog buffer to the redolog file

when one third of redo log buffer is filled.
when transcation is completed.
time out of 3 sec.

1.Every 3 secs
2.At commit(when a tx commits)
3.When one third of redo log buffer is filled.
4.When there is more than 1 Mb of redo entries(redo data or
changes) in the redo log buffer
5.Before DBWR background process writes dirty buffers from
db cache to datafiles(database)

At any of the above instances(cases) lgwr writes the redo
entries or redo data or changes from the redo log buffer to
the redo log file.

program global area is used to copy a block from the data
file to the buffer cache. 
two processes are present in the pga:dedicated server
process and the shared server process.

Program Global Area (PGA) is a memory buffer that contains 
data and control information for a server process

program global area 
allocated space for server process

The System Global Area (SGA) is a shared memory region
allocated by ORACLE that contains data and control
information for one ORACLE instance. 

this writes all the commited data from the buffer cache to
the  data file.

DBWR write to database when checkpoint occurs

when log switch occurs
shut down 
every 3 sec if set
when treshold is full
least recently used

The DBWn background process writes to the datafiles 
whenever one of the following events occurs:
 A user’s Server Process has searched too long for a free 
buffer when reading a buffer into
the Buffer Cache.
 The number of modified and committed, but unwritten, 
buffers in the Database Buffer
Cache is too large.
 At a database Checkpoint event. See Chapters 10 and 11 
for information on checkpoints.
 The instance is shut down using any method other than a 
shutdown abort.
 A tablespace is placed into backup mode.
 A tablespace is taken offline to make it unavailable or 
changed to READ ONLY.
 A segment is dropped.

Database writer writes modified blocks from the database buffer 
cache to the data files.

Database Writer (DBWn) writes the dirty buffers from the 
database buffer cache to the data files.

Redo Log Buffer stores redo entries a log of changes made to
the database. 

pmon, smon, dbwr, lgwr, ckpt, arch, disp,cjq and more...

there aretwo types of oracle background process
1.mandatory background process
  DBWn,PMON,SMON,LGWR and  CKPT
2.optional Background process
  ARCn,CJQ0,LMS,Dnnn,Pnnnn,Snnn,LCKn,RECO etc.

RECOver (RECO) is used to resolve distributed transactions
that are pending due to a network or system failure in a
distributed 
database. At timed intervals,the local RECO attempts to connect 
to remote databases and automatically complete the commit or 
rollback of the local portion of any pending distributed
transactions. 

when ever a commited data is written from the buffer cache
to the datafile , a checkpoint occurs.

SGA and ORACLE background processes constitute an ORACLE
instance. 
(or) 
Combination of memory structure and background process. 

Oracle Instance constits of on 

         Memory Structure and Background Process

* Memory structure constists of SGA and PGA

 =>SGA constists of 
  1)Shared Pool
  2)Database Buffer Cache
  3)Redo log Buffer
  4)Java Pool
  5)Large Pool
 
 =>PGA constists of 
  1)Server Process
  2)User Process

* Background Process constists of 
 
  1)PMON
  2)SMON
  3)DBWR
  4)LGWR
  5)CKPT
  6)Others
  
 and optional Bg process also there
 RECO,QMNn,ARCn,LCKn,LMDn,Snnn,Pnnn etc..

Atleast one Dispatcher process is created for every
communication protocol in use. 

Cursor is a private pl/sql area to hold the result set of 
multiple records. to process or perform any DML operations 
on these records.

Cursor is a programmer created sql area where the multiple 
records are processed into the PL/SQL block.

Server Process handle requests from connected user process. A 
server process is in charge of communicating with the user
process and interacting with ORACLE carry out requests of
the associated user process.