The Micro Four Thirds system is a revolutionary digital hybrid camera system which combines compact size, ease of use and movie mode functionality of the digital point and shoots with the larger sensor (and thus higher image quality and shallower depth of field) and interchangeable lens capability of digital SLRs.
It achieves this by removing the mirror of the SLRs, and is only possible with the rapid technologic changes that have dramatically improved contrast detect AF functionality on par with entry level dSLR AF speed, while electronic viewfinder technology has also improved.
It’s short lens flange to sensor distance (20mm compared with 40mm on Olympus Four Thirds dSLR system) means that super wide angle lenses can be made much smaller (see the 7-14mm zooms here) and one can adapt almost ANY lens every made on it, including Leica M, Leica R, Olympus Pen, Olympus OM, Canon FD, Nikon F, Minolta, Pentax K, Hasselblad, Pentax 6×7, and even Canon EOS lenses can be used with aperture control and some AF!
Focal reducer adapters allow full frame lenses to be used with only 1.4x crop instead of 2x crop and you gain 1 stop more aperture plus improved image quality.
The system is thus THE MOST ADAPTABLE camera system available in terms of legacy lenses – however, they will generally only be able to be used in manual focus, and in 35mm terms, the sensor gives a 2x crop factor so that the angle of view of a 50mm lens will be similar to that of a 100mm on a 35mm film camera.
The system is by far the most mature of the mirrorless camera systems with a multitude of excellent lenses and camera bodies to choose from - see the Olympus OM-D system and how to make the most of it for more tips and ideas on how to use the Olympus cameras.