SGA

SGA is made up of the following -

Fixed - Small and always constant depending on platform, os and database version
Variable (large pool, shared pool, java pool)
Database Buffer
Redo Log Buffer

10G
Oracle introduced memory management. Basically you set the following parameters and oracle manages the memory for you. The theory is the database will adjust the memory components as on a as required basis.

sga_taget <> 0
statistics_level = TYPICAL