A program global area (PGA) is a memory region that stores the data and control information for the server processes. Each Server process has a non-shared memory created by Oracle when a server process is started. Access to the PGA is exclusive to that server process and it is read and written only by Oracle code acting on its behalf. Broadly speaking, PGA contains a private SQL area and a Session memory area.
A private SQL area contains data such as bind information and runtime memory structures. Each session that issues a SQL statement has a private SQL area.
Note that the Location of a private SQL area depends on the type of connection established for a session. If a session is connected through a dedicated server, private SQL areas are located in the server process’s PGA. However, if a session is connected through a shared server, part of the private SQL area is kept in the SGA.
Session memory is the memory allocated to hold a session’s variables (logon information) and other information related to the session. For a shared server, the session memory is shared and not private.---
The above text is an excerpt from:
Oracle 10g Grid & Real Application Clusters
Oracle 10g Grid Computing with RAC
by Mike Ault, Madhu Tumma