I recently acquired a four/thirds camera. The one I bought is the Panasonic Lumix DMC-G3. I began with Pentax 35mm cameras in the 1970's; moved to Canon equipment in the 90's; then switched to digital cameras about 15 years ago.
The four/thirds sensor in the Panasonic camera is smaller than the APS-C sensor in many digital SLR cameras, but it is significantly larger than the sensor in point and shoot digital cameras. This means that the image quality can be close to that of the APS-C cameras, but the body and lenses can be made smaller and lighter.