The SYSTEM global area is a group of shared memory area that is dedicated to oracle instance. All oracle process uses the SGA to hold information. The SGA is used to store incoming data and INTERNAL control information that is needed by the database. 

You can control the SGA memory by SETTING the parameter db_cache_size, shared_pool_size and log_buffer.

Shared pool portion contain three major area: Library cache (parse SQL statement, CURSOR information and execution plan), data dictionary cache (contain cache, user account information, privilege user information, segments and extent information, data buffer cache for parallel execution message and control structure.

The system global area is a group of shared memory area that is dedicated to oracle instance. All oracle process uses the SGA to hold information. The SGA is used to store incoming data and internal control information that is needed by the database. 

You can control the SGA memory by setting the parameter db_cache_size, shared_pool_size and log_buffer.

Shared pool portion contain three major area: Library cache (parse SQL statement, cursor information and execution plan), data dictionary cache (contain cache, user account information, privilege user information, segments and extent information, data buffer cache for parallel execution message and control structure.