They are documented in the Database Reference at http://tahiti.oracle.com.

There are a lot of system views involved, and they are generally prefixed with USER_ (objects that you own), ALL_ (objects that you have access to) and DBA_ (all objects).

Common views you'd use frequently would be user_tables, user_indexes, user_tab_columns, user_source, user_segments, user_objects, user_tablespaces etc.. See the docs for more details.