Apparently aside from the actual mounts the mirror less cameras have a different amount of distance the light must travel between the lens and the sensor. That is what makes them smaller.
That is correct. In the history of cameras, the SLR (Single Lens Reflex) design allowed the photographer to see through the lens that was taking the photo, instead of a second lens (TLR) or viewfinder (rangefinders and others). This was a big advantage for some kinds of photography and quickly caught on with the public and professionals.
However, in order that the light from the lens could be seen by both the photographer and the film (nowadays a sensor instead), there had to be some way of splitting the light or redirecting it as needed. The SLR design used a mirror that flipped down to block the film but send the light to the viewfinder until the shutter was pressed; then the mirror flipped up out of the way, the shutter curtain opened, and the film could be exposed. It all happened very fast; just the blink of an eye for the most part. But the viewfinder was dark until the mirror flipped down again.
Some camera makers experimented with ways to split the light optically so it went to both places all the time; they used a special prism instead of a flipping mirror. But splitting light means each destination gets less light, so it required darker viewfinders and high-speed film for the same exposures. Not really a big seller for the most part.
The first digital SLR cameras (DSLR) worked the same way as the older SLR. In fact, Sony is now experimenting again with prisms to split light like some did in the past on their Alpha series cameras. I don't know if it will catch on or not, but they're reviving the concept.
DSLR cameras mostly have smaller sensors than a traditional film camera. The current standard (not all use it, but many do) is 'APS-C'. This is about 1.5 times smaller than the traditional 35mm film camera frame size. So if you mount an older lens designed for film on a newer DSLR camera, it will have a 'crop factor', meaning it will behave as if it were a longer lens. A 50mm lens will behave like a 75mm lens, and so on.
The newer EVIL cameras have a new concept made possible by electronics in place of optics. They remove the optical viewfinder and replace it with an electronic (LCD or OLED) viewfinder, or they do away with the viewfinder completely and just have an LCD on the back of the camera.
With less 'stuff' in between the lens and the sensor (flipping mirror, prism, etc), they can (and did) make the body more compact. The critical measurement is called the 'flange to film' distance; an archaic term, but it essentially means the distance from the rear of the lens mount to the film plane (now a sensor instead), and it is measured in mm.
Adapters are possible when the older camera lenses have a longer registration distance (another way of saying flange to film) than the new camera. So with the new EVIL cameras, many old manually-focused lenses are finding a new lease on life. Not everybody cares about this - manual lenses are a PITA to work with in some cases, and newer auto-focus lenses are quite nice, nothing to sneeze at. But some just like the older lenses. They are often 'faster' lenses (wider apertures) which allow some special effects with background blurring and that sort of thing, and some consider them still superior optically to a lot of modern lenses. Some of us just have a lot of them rattling around and would like to put them back to use again.
There are essentially three competing systems for smaller EVIL cameras that have interchangeable lenses. The micro 4/3 standard is very nice and is becoming very popular, several manufacturers use it. However, the sensor is even smaller than APS-C, so the 'crop factor' with older manual focus lenses is 2x, meaning a 50mm lens acts as if it were a 100mm lens. This is nice if you want to do a lot of telephoto work, but not if you want to do wide-angle, because there are not a lot of older manual focus lenses out there that are still considered wide-angle after being multiplied by 2!
Sony also has a system for EVIL, and it uses the APS-C size sensor. So older manual focus lenses mounted on it have the same 1.5 crop factor as most DSLR cameras.
And Samsung has also introduced some EVIL cameras, also with APS-C sized sensors, and with yet a different style mount.
So it seems the market is still developing (no pun intended) for smaller mirrorless interchangeable lens cameras. I have invested in the Pentax DSLR system, which lets me keep all my older Pentax lenses, autofocus and manual focus, from my old Pentax film cameras, so that's a win for me. But I'd love to put my older Canon FD mount lenses back into service, and Canon, unlike Pentax, moved from the FD mount to a totally incompatible mount (EOS or EF) a long time ago, leaving all my old lenses as orphans. You can get adapters to use Canon FD lenses on Canon EOS bodies, but they are crap (my opinion, some people like them); they have an optical lens inside to retain infinity focus and it degrades the images.
So of the three 'systems' out there for mirrorless cameras, I'm currently learning towards Samsung. The rest of them are also outstanding systems, probably better than the Samsung in many ways, but I think this will fit my personal needs best. Haven't decided yet, though. Mind still open.