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drop

A truncate table or truncate cluster statement cannot be used on a synonym.

drop tablespace including contents

This command will remove the tablespace, its datafiles and the objects (segments) from the data dictionary.

drop table t_foo

With Oracle 10g and Flashback DB turned on, a drop table doesn't drop the table but renames it. Also, indexes and other structures remain. This makes it possible to quickly recover from human errors.
Dropped objects can obviously be seen in dba_recyclebin.
Alternatively, use show recyclebin in SQL*PLUS.
See also drop table.

Definitely dropping a table

In 10g, a table that is dropped goes into the recycle bin from where it can be recovered. In order to definitely drop (that is: purge) a table, use the following syntax: 
drop table t_unused purge
See also _recyclebin.