-- to find out system privileges granted to user 

SQL> select privilege from dba_sys_privs where grantee='THIRU';

PRIVILEGE
----------------------------------------
ALTER SESSION
UNLIMITED TABLESPACE

-- to find out roles granted to a user

SQL> select GRANTED_ROLE from dba_role_privs where GRANTEE='THIRU';

GRANTED_ROLE
------------------------------
DBA
CONNECT
RESOURCE
TEST_ROLE

-- to find out explicit table privileges granted

SQL> select owner,table_name,PRIVILEGE from dba_tab_privs where GRANTEE='THIRU';

no rows selected

-- to find out privileges contained within a role

SQL> select privilege from dba_sys_privs where grantee='CONNECT';

PRIVILEGE
----------------------------------------
CREATE VIEW
CREATE TABLE
ALTER SESSION
CREATE CLUSTER
CREATE SESSION
CREATE SYNONYM
CREATE SEQUENCE
CREATE DATABASE LINK

8 rows selected.

-- also system privileges contained in a role

SQL> select PRIVILEGE from role_sys_privs where ROLE='CONNECT';

PRIVILEGE
----------------------------------------
CREATE VIEW
CREATE TABLE
ALTER SESSION
CREATE CLUSTER
CREATE SESSION
CREATE SYNONYM
CREATE SEQUENCE
CREATE DATABASE LINK

8 rows selected.

..and so on.