In the database management systems developed by the Oracle Corporation, the System Global Area (SGA) forms the part of the system memory (RAM) shared by all the processes belonging to a single Oracle database instance. The SGA contains all information necessary for the instance operation.
System Global Area Wikipedia
In general, the SGA consists of the following:buffer cache: holds copies of data blocks read from datafiles
dictionary cache: holds information about data dictionary tables, such as information about account, datafile, segment, extent, table and privileges
redo log buffer: contains information about database transactions, both committed and uncommitted, in preparation for writing to online redo log files
shared pool: holds the dictionary or row cache, the library cache, cursor definitions and shared SQL.
Java pool: holds information for parsing Java statements.
large pool: including the User Global Area (UGA))
stream pool:this is remote connectivity for the database.
log buffer:this is cirucular ratiation and modify or change all data in database.
From Oracle Database version 10g, Automatic Memory Management (AMM) allows simplified and dynamic configuration of the SGA.