Real Photo Postcards RPPC
"Real photo postcards, also known as RPPC, are a very specific type of picture postcard. Real photo postcards are true photographs produced chemically from negatives onto photographic paper. Other than the postal markings on the back of the images and the standard postcard sizing of 5 1/2" x 3 1/2", real photo postcards are real photographs with postcard backs, produced in limited quantities and providing a remarkable view of ordinary life in the early years of the 20th century."
There is lots more to learn about these cards before you start spending serious money for them. However, the list below will help you get started with what is available, what to look for in auction descriptions, and lots of other good information. If something is cheap it is probably worth taking a chance and buying it. Even if it isn't worth much it will help educate you about RPP.