I will jump in here since I now use the Panasonic G-1 for all my art photography as well as studio work for the manuals and books. The names four-thirds, APS-C and others are hang overs from the days of video tubes and now refer to the sensor size (a four thirds sensor is half the size of 35mm film, an APS-C sensor is about 5/8 the size of 35mm film, etc.). In the case of the four-thirds standard promulgated by Olympus, Panasonic and Leica it also refers to the lens mount. Micro four-thirds is a different, smaller lens mount on a camera which uses the same 4/3 sensor size where the mirror box and penta prism have been removed allowing the lens mount to get much closer to the sensor (a shorter back focus distance in camera terms). This shorter back focus distance allows older lenses designed for much longer back focus distances required to miss the SLR mirror flipping up and down to be mounted on the micro four-thirds body and still focus to infinity by the use of a simple adapter.
Olympus, Panasonic and Leica all offered their brand digital cameras based on the original four-thirds standard with the very handy live view mode for composing on a rear mounted LCD. That well integrated live view mode made it much easier to do studio work since you no longer had to peer into a small view finder from an awkward angle as studio work often requires. For a long time I used the Leica version for much of my work and Leica designed (still does) many of the lenses for Panasonic.
Panasonic was the first of the trio to replace the mirror box and penta prism with a high resolution electronic view finder out of their professional video cameras allowing the use of the shorter back focus micro four-thirds mount. That is called a Panasonic Lumix G-1 or GH-1 if you want high resolution video as well. Olympus has recently added their own micro four-thirds version and many others are rumored to be developing their own versions.
What makes the micro four-thirds such a hot topic in the photo industry is that short back focus distance between the mount and the sensor. Adapters are flooding in from Europe and the far east to allow mounting of nearly any 35mm or 16mm movie camera lenses from virtually any legacy lens mount such as Leica screw, M and R mount lenses, Voigtlander, Zeiss, Schneider, Nikon, Canon EF and current mounts, Olympus OM, and many others that have a mechanical aperture ring. All manually focus and stop down and require shooting in either manual or Aperture Preferred mode but the results are stunning. As good as the new digital designed zoom lenses are, they are no match for a premium maker of what we used to call "prime" lenses such as the Leica 50mm F2.0 Summacron or Canon 50mm FD mount F1.4 lens. Mounted on a Panasonic G-1 body with the right adapter the images are remarkable.
One Shutterbug magazine editor covering the recent Tokyo vintage camera show (largest in the world I'm told) started his column by noting that nearly everyone entering the show had a Panasonic G-1 slung over their shoulder. The G-1 single handedly doubled or tripled the eBay price of the older high quality legacy lenses.
Hope this helps.
When you do you might want to take a look at the micro four thirds cameras. Great liitle cameras and they can use all the older Canon, Nikon, Leica, and other brand manual lenses (w/ manual focus and aperature) with the appropropriate adaptor. The Panasonic G1 or GH1(if you're interested in stills and HD video) have great image quality and are still small enough that I'm more likely to carry it around than a Canon or Nikon.
Ok, Fred. I realy am out of the looooooooooooooooooooooooop here. What are the micro four thirds?