SLR cameras were designed for the film era. The "reflex" refers to the mirror inside, which redirects light to the viewfinder and then flips out of the way when the shutter is fired, letting the light fall onto the film. Because of the mirror, the body of an SLR is relatively large, and because the lenses are so far away from the film plane (or, these days, the sensor chip), they have to be big, too. Look at the size difference between a compact camera and an SLR for an instant example of this.

Micro Four Thirds does away with the mirror, making the camera much smaller. The gimmick is that you can still change lenses, just like an SLR. And because the sensor size is standard across Micro Four Thirds cameras, the confusion of focal length multipliers disappears (although if you do want to know the 35mm equivalent, just times multiply by two), and you you don’t have to sell all your glass if you swap from one camera brand to another.