Kamisori - translating literally to 'razor'- is the Japanese equivalent of the folding straight razor that most folks in the west are familiar with. While they are similar in principle, those who have shaved with both know that they are worlds apart.
Kamisori are single-beveled, meaning that one side is sharpened to a typical angle, and the other side is set at 0 degrees, even slightly concave. This means that different angles are required on either side of the blade. The kamisori design makes them incredibly ergonomic and offer a more natural feel, like working with a kitchen knife or a pen. The Japanese forging gives them superior edge retention to their German counterparts, and the shorter blades make them popular among many folks, especially those who wish to trim their beards.
As in many countries, folks in Japan have been shaving for a long time. For as long as there has been steel used to make swords, spears and tools, there have been straight razors in some form or another. Over many centuries, the Japanese razor eventually took the form that we have today: the kamisori.