SGA (System Global Area) is a memory area allocated during an instance start up.
SGA is allocated as 40% of RAM size by default.
SGA size is controlled by DB_CACHE_SIZE parameter defined in initialization parameter file (init.ora file or SPFILE).
PGA (Program or Process Global Area) is a memory area that stores a user session specific information.
PGA is allocated as 10% of RAM size by default.