Ever since the SLR (Single Reflex Lens) began to take the digital path (DSLR), the weight of the camera increased tremendously because of the components that needed to be included with the camera for the digital sensors to work, specially the digital signal process. As a result, DSLR was getting bigger and heavier. Even though there were many efforts to bring out lighter DSLRs, but that was at the expense of features like speed, quality and above all, ease of use.
M 4/3
Big and heavy was the standard rule until one day Olympus and Panasonic got together to create something evolutionary, the M 4/3. So what is it? Micro Four Third (M 4/3) is a system in which instead of having the pentaprism and the mirror to reflect the image to be seen by the sensor, it would use the sensor right away to provide a live view of the picture. Now this made way for smaller and lighter designs by removing the 2 elements which wer not needed by the digital sensor.
Practical Implementation of the M 4/3 system

Then Olympus released an update to it, the E-P2 which resolved some of the problems of the drawbacks of the E-P1. Finally, they released a low-price model, the Olympus PEN E-PL1.
Panasonic too came up with their own model, the Panasonic GF1 based on the M 4/3 design.
The Latest Entry

More information about these two cameras can be found here.