Oracle metadata

Wiki
http://en.wikipedia.org/wiki/Oracle_metadata

The Master Key to Oracle's Data Dictionary
http://www.oreillynet.com/pub/a/network/2002/10/28/data_dictionary.html
USER_
USER views return information about objects owned by the currently-logged-on database user.

ALL_
ALL views return information about all objects to which you have access, regardless of who owns them.