Hello Mayura Sawant,
To add to what Giridhar SK mentioned, you must have seen postal envelopes with the stamp denominations NOT in actual stamps but sort-of stamped in RED or BLACK which includes the postal amount. No separate stamps are needed to be affixed for such envelopes. THIS METHOD IS CALLED FRANKING. This is usually adopted for Bulk postings--like magazines, letters from companies to shareholders, etc.
Franking is done by any authorized Government body. Sometimes private companies are also allowed to do it--subject to strict control by the respective Govt Dept--since they have to pay the equivalent cash for the total amount franked [the Franking machine has a counter to show how much worth of money has been franked].
Hope this clarifies your query.
9th June 2011 From India, Hyderabad