History Of Postage Stamps
Before the introduction of postage stamps, mail were delivered and paid for by the receiver. This led to a variety of problems such as when the receiver was unable or unwilling to pay for the delivery service. It also allowed misuses of the system such as senders delivering pieces of mail just to annoy the receiver and making them pay for it at the same time (much like the problems of spam today). The postal system itself was also rocky, with mail often being lost or severely delayed. Plagued by problems, the postal system clearly needed a reform very badly.
The answer came in 1840 in the form of postage stamps. Instead of requiring the receiver to pay the postage for mail they may or may not want, the burden is shifted to that of the sender. If you were living in Britain at the time, you would have had to buy a penny black stamp and affix it to your package using glue (it's not self adhesive like today's stamps are), and drop it off at the post office.
This dramatically helped help the postal system reform as people are no longer bombarded by random pieces of mail. The post office also got more money since people were paying up front, which allowed them to deliver mail more efficiently and on time.
It wasn't long before other countries followed suit. Switzerland issued their own Zurich stamps (costing 4 and 6 rappan respectively) only 3 years later in 1943, and Brazil their Bull's Eye stamps later that year. Today, postage stamps are used to prepay for letters and packages throughout the world.