Difference between TRUNCATE, DELETE and DROP commands in database

DELETE command is used to remove rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. 

TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUCATE is faster and doesn't use as much undo space as a DELETE.

The DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.

DROP and TRUNCATE are DDL commands, whereas DELETE is a DML command. Therefore DELETE operations can be rolled back (undone), while DROP and TRUNCATE operations cannot be rolled back.

TRUNCATE is much faster than DELETE.

Reason:When you type DELETE.all the data will get copied into the Rollback Tablespace first, then delete operation will get performed. That's why when you type ROLLBACK after deleting a table ,you can get back the data(The system get it for you from the Rollback Tablespace).All this process take time. 

Reference: 

http://www.orafaq.com/faq/difference_between_truncate_delete_and_drop_commands