SGA is used to store shared information across all database users.It mainly includes Library cache, Data Dictionary cache, Database Buffer Cache, Redo log Buffer cache, Shared Pool.
Library cache - It is used to store Oracle statements.
Data Dictionary Cache - It contains the definition of Database objects and privileges granted to users.
Data Base buffer cache - It holds copies of data blocks which are frequently accessed so that they can be retrieved faster for any future requests.
Redo log buffer cache - It records all changes made to the data files.