what is oracle Data Dictionary

Read only set of tables provides Administrator metadata about DB.
e.g columns,index,constraint (user can only read it)
write priv is to do by oracle itself

two forms

1>tables and views(db structure)----remains static,only changes when we creat something
three set of views
user_= users view(wha you own,what is in your schema)
all_=expanded users views (what you can access)
dba_=dba's view(what is in everyone's schema)
#select table_name from dba_tables wher table_name like '%$";
to see the base tables to which the views refering to.

#desc ds$;
#desc dba_tablespaces and compare the output

2>dynamic performance data(starts with v$ )

e.g v$session,v$database