Explain Oracle memory structure.

The Oracle RDBMS creates and uses storage on the computer hard disk and in random access memory (RAM). The portion in the computers RAM is called memory structure. Oracle has two memory structures in the computers RAM. The two structures are the Program Global Area (PGA) and the System Global Area (SGA). 
 
The PGA contains data and control information for a single, user process. The SGA is the memory segment that stores data that the user has retrieved from the database or data that the user wants to place into the database.

PGA Program global area Contains  
Session variables ,Stack space,Cursor status ,Sort Areas

SGA(system global area) & PGA(program global area) are two memory structures in oracle.
SGA::is used to store shared information.It having Buffercache,Redolog buffer,Shared pool,java pool,Larg pool and stream pool.
PGA::is used to store session specific information.i.e sorting,merging information stored in pga.