Here is a great British article about how U.S. stamps are made.

Some history of printing stamps: Internal Revenue stamps and postage stamps were a big part of the Bureau of Engraving and Printing’s history.  The BEP began producing revenue stamps in 1866.  These stamps were used well into the twentieth century to collect excise taxes.  Producers of goods subject to excise taxes were required to place a Government stamp, known as a revenue stamp, on their product to prove that the tax had been paid.  Postage stamp production started at the BEP in 1894 and continued until 2005 when production was given to private printing firms. (Bureau of Engraving)