Sigma introduced a new concept that has been taken up by other companies, such as Olympus, Panasonic, and now Leica with the X1. This is referred to as the 4/3 format or system, and it seems to be getting some traction in the market. Sigma decided to build a compact camera with a large sensor, about equal in size to the Prosumer/Amateur DSLR sensor. The origial film Leica may have inspired that thought. It was always a small 35mm range-finder system that weighs a fraction of that of larger SLRs or DSLRs. The digital four-thirds cameras have no optical view finder, but expect the LCD on the back to be the sole viewing element for the photographer. The result is a compact camera without mirror or prism to facilitate an optical viewfinder, permitting smaller lenses, but with a large and high quality sensor. You can carry the whole system in a small bag or even in your pockets, as Kevin Spacey demonstrates here in the amusing video for the Olympus Pen (4/3).

Note: because of the sensor size, which is a little smaller than DX, you will have to double the focal legth of the micro 4/3 lenses (e.g. a 20 mm lens is actually the equivalent of a near-normal 40mm lens and so on).