The Leica M mount is the oldest lens mount currently in widespread use, predating the Nikon F mount (and the modern SLR camera) by several years. Designed for Leica rangefinder cameras, which do not feature a mirror between the lens and the image sensor (film, originally), the M mount places lenses in close proximity to the sensor. Rangefinder cameras were the predominant mirrorless interchangeable lens cameras for 35mm analog photography.
The short sensor-to-lens distance of rangefinder cameras enables M-mount lenses to be designed and built in physical dimensions much smaller than their SLR lens counterparts. As such, M-mount lenses are the perfect small manual focus companions to today's mirrorless cameras, including Sony E mount and Micro Four Thirds system bodies.
Since the M-mount has always been featured on cameras with a 24mm x 36mm imaging sensor area, these lenses will provide image circle coverage for any sensor size up to the contemporary "full frame" size of 24mm x 36mm. They can be used with simple manual adapters, available at Hot Rod Cameras.