There are basically 9 Processes but in a general system we need to mention the first five background processes. They do the house keeping activities for the Oracle and are common in any system.
The various background processes in oracle are
Data Base Writer (DBWR): Data Base Writer Writes Modified blocks from Database buffer cache to Data Files. This is required since the data is not written whenever a transaction is committed.
Log Writer (LGWR): Log writer writes the redo log entries to disk. Redo Log data is generated in redo log buffer of SGA. As transaction commits and logs buffer fills, LGWR writes log entries into an online redo log file.
System Monitor (SMON): The System Monitor performs instance recovery at instance startup. This is useful for recovery from system failure
Process Monitor (PMON): The Process Monitor performs process recovery when user Process fails. PMON Clears and Frees resources that process was using.
Check Point (CKPT): At Specified times, all modified database buffers in SGA are written to data files by DBWR at Checkpoints and Updating all data files and control files of database to indicate the most recent checkpoint
Achieves (ARCH): The Archiver copies online redo log files to archival storal when they are busy.
Recover or (RECO): The Recovered is used to resolve the distributed transaction in network
Dispatcher (Dnnn): The Dispatcher is useful in Multi Threaded Architecture
Lckn: We can have up to 10 lock processes for interred instance locking in parallel SQL.
