http://www.dba-oracle.com/concepts/sga_system_global_area.htm

http://www.techopedia.com/definition/27187/system-global-area-sga

The System Global Area in an Oracle database is the area in memory to facilitate the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database. The structure is database buffers, dictionary cache, redo log buffer and shared pool area.

thanks for answering,,,,