Q:

What information is stored in a context area in plsql? 

I learnt that context area in oracle is a private memory space where information needed for processing SQL statements are stored.  However, I didn't get anywhere knowledge about what information is stored there. 

A:

Oracle has a default context named 'userenv' which stores a great deal of information about the session.  We can access the information using sys_context().  For instance, to get the database name...
select sys_context('USERENV', 'DB_NAME') from dual;

All of the valid parameters are listed in the SQL Reference for sys_context here.  
Some very useful parameters are 'ACTION', 'MODULE' and 'CLIENT_INFO'.  These fields are populated through dbms_application_info calls, which allows us to pass information between programs.  
However, we can extend this capability further by defining out own context namespaces.  This requires the CREATE CONTEXT privilege (by default only available to DBA users).  Programs write to and read from context using dbms_session calls.  With our own context namespace we can stash bespoke information in session memory.  Fine-Grained Access control (row-level and column security) relies on this feature.  So there's a lot more about contexts in the Security Guide. 