To my knowledge, when delete statement is issued the data is actually deleted from the blocks and the space in the blocks will be realeased. But this space can be utilized when the pctused level has been reached.
Also, Delete operations are slow(As it acts on blocks directly)

Could any one tell me how the Truncate command actually work.. Because, truncating at table as big as 25Million records take very few milliseconds while actual delete takes hours.