Way back when, 35mm was classified as "miniature" format, so the cameras were "miniature cameras" and so forth. So anything smaller than that was sub-mini. (At least until the 70s and after, when 110 and disc cameras came out, but never got the submini label).
Anyway, this is the submini camera. It's the size of a slim candy bar. It produces an image 8 x 11mm on a slim strip of roll film without sprocket holes. The film is loaded in cartridges so setting up the camera is easy, though I wouldn't want to develop the stuff in my darkroom.
The camera has an interesting history. It was developed by a Lett photographer and initially produced in Riga, Latvia, just prior to and well into World War II. After the war, production resumed in Wetzlar, West Germany (though the Russians are said to have made their own during the Soviet era). Minox is still in business today.
The early versions were given roman numeral designations, then they shifted to letters. This is the ubiquitous B model, the most popular and longest-lived of the bunch. The big difference is the integrated light-meter (on top here), whereas earlier models had clip-on meter accessories.
Mine came with the case and measuring chain; the latter doubles as a carrying lanyard, but is also meant to set the camera at the proper distance to reproduce letter-sized documents; that's a holdover from its days as a spy camera. Cynics believe that Minox played up the spy camera stuff as a marketing gimmick, but Minox cameras were used as spy cameras on various occasions: they're small and easy to conceal, they're simple to operate, and they can copy documents easily enough.
This is my only submini (unless you count my Kodak 110 and Disc cameras), but there are collectors who specialize in them. Besides the Russians, Yashica and Fuji made Minox-compatible cameras, and there are a number of other very-small format cameras of various makes. The heyday for these is around 1960, the Sputnik-era, and I suppose there were a lot of junior James Bonds running around.
Camera manual: Orphan Cameras.com
Modern Photography magazine camera test: November 1958