Oracle Instance Structure

A database instance is a set of memory structures that manage database files. It includes a memory area (SGA) and several background processes.

The most important SGA components are the following:
Database buffer cache is the memory area that stores copies of data blocks read from data file.
Redo log buffer is a circular buffer that stores redo entries describing changes made to the database.
Shared pool caches various types of program data, such as parsed SQL, PL/SQL code, system parameters and data dictionary information.
Large pool is an optional memory area intended for memory allocations that are larger than is appropriate for the shared pool.
Streams pool stores buffered queue messages and provides memory for Oracle Streams processes
Fixed SGA is an internal housekeeping area.

Background processes run by default in a database instance started.
PMON monitors the other background process and performs process recovery
SMON is in charge of a variety of system-level cleanup duties.
DBWn writes the contents of database buffers to data files.
LGWR manages the redo log buffer.
CKPT updates the control file and data file header with checkpoint information and signals DBWn to write blocks to disk.
MMON performs many tasks related to the Automatic Workload Repository.
REC0 automatically resolves failures in distributed transactions.