Oracle Database architecture on Windows

An Oracle instance is shown as a Windows service (a background process registered with the operating system). The service is started by Windows and requires no user interaction to start. It contains the memory structures and threads needed to access the database.
The client connects via a shadow thread to the Oracle instance. The shadow thread connects directly with the process monitor. The memory structures include a shared pool of a library cache and a data dictionary cache, a database buffer cache, and a redo log buffer.
The threads include the database writer, log writer, process monitor, system monitor (all required while the database is available), checkpoint process, archive process, and distributed recovery process (all optional while the database is available).
There is an Oracle Database containing data files, control files, and redo log files. Associated with the database is the parameter file, password file, and archived log files.

Oracle Database Thread	Description	Required/Optional
DBW0	database writer	Required
LGWR	log writer	Required
PMON	process monitor	Required
SMON	system monitor	Required
CKPT	checkpoint process (thread on Windows) that runs by default on Windows	Optional
ARCH0	archive process (or thread on Windows)	Optional
RECO	distributed recovery background process	Optional

Note: You can view running background processes by issuing the following query: SQL> select * from v$bgprocess where paddr <> '00';