Creating Unrecoverable Tables

You didn't mention version, my is 8.1.7.

You can create an unrecoverable table with deprecated clause UNRECOVERABLE or newer clause NOLOGGING while creating table.
The creation of the table is not logged in redo logs so you can loose it if you don't make backup.
Because of no redo log generation it can save space and time but it makes sense only if you create the table with CREATE TABLE AS SELECT ... statement or if you fill it with DIRECT-LOAD-INSERT: INSERT /*+ APPEND */ INTO nologging_table SELECT ....
or if you use SQL*Loader.

Other DML are fully logged.

"No redo log generation" means almost no generation, some redo is always generated for dictionary changes.

More you can find in the manual : http://otn.oracle.com/docs/products/....817/index.htm