In traditional front illuminated CMOS sensors, there is a significant loss in light due to the circuit layers that need to be placed next to the photo diodes. In Back-Side Illuminated (BSI) sensors, these circuit layers are printed at the back surface of the same substrate so that it does not interfere with the light capturing efficiency of the sensor.
A stacked CMOS sensor, on the other hand, is one on which the micro lenses, photo diodes, circuit layer and image sensor are all stacked together. This improves the efficiency and speed of signal processing. The latest stacked sensors from Sony (designated as Exmor RS) includes a DRAM circuit (integral memory) in the stack so that it acts as a buffer to store the signals before it is processed.