Sorry about that, cojiro.
As sybaseguru explained, a schema is sort of like a box of everything you own - tables, views, indexes, synonyms, packages, procedures, sequences, etc, etc, etc.
There are also three basic levels of views in the data dictionary that people normally look in to see what's in all of the boxes. USER will show you everything in your box. ALL will show you anything that you have privileges on in any box. DBA will show you everything in every box - but (as DBA would imply) this is normally reserved for specific, high-powered users.
So if you want to see all of the tables in your box, you would look in USER_TABLES; all of your triggers would be in USER_TRIGGERS, and so on.
If you wanted to see all of the views that are available to you, you would look in ALL_VIEWS.
If you had proper authority and wanted to see every index in the database, you would look in DBA_INDEXES.
Hope this helps.