DBA

export ORACLE_HOME=/products/oracle/ora9i
export ORACLE_SID=testdb

sqlplus '/ as sysdba'

Tables

To get the list of accessible tables

 SELECT table_name FROM all_tables;
           OR

A user grants access to his table to another user and then that other user creates a synonym in his schema. Then simply type

 SELECT SYNONYM_NAME FROM USER_SYNONYMS;

To get the list of tables that you possess

 SELECT TABLE_NAME FROM USER_TABLES ;

To get the list of views that you possess

 SELECT VIEW_NAME FROM USER_VIEWS ;

Users

To list the users

  SQL> select username, user_id from dba_users;

To list the users who own objects

 SQL> select distinct(owner) from dba_objects;

To list the counts of object types by owner

 Sql>  select  owner, object_type, count(*)
       from    dba_objects
       group by owner, object_type