It looked like this (only smaller).
The lady in the image is Queen Victoria, the place was Britain, and the man who did all the administration and arrangements was Rowland Hill.
Who invented the idea of the stamp is hotly contested (some of these stamp collectors are men of high passions, you know) but William Dockwra and Robert Murray established a pre-paid London penny post in 1680 where proof of payment came in the form of a design hand-stamped on the letter or package. It looked like this:
This seems to be why a stamp is called a stamp.
Other inventors of the idea of the postage stamp include the Slovenian Lovenc Košir and the Scot James Chalmers - but it was Rowland Hill who got all the paperwork and persuasion and admin done and finally got the idea adopted.
The new (outside London) pre-payment system was a great advantage to the people delivering the letter: first, payment was guaranteed; and, second, the sender had an incentive to restrict the size and weight of the item to be sent.
In fact it meant that letters became so easy and cheap to send that even quite poor people could keep in touch: and so my grandfather, in about 1910, could send a postcard to his sweetheart, my grandmother, which simply said see you tonight.
Word To Use Today: stamp. The Old English word was stampe, which referred of course to the foot sort of stamping.