The sensor is to a digital camera as film is to an analog camera. A camera with a larger sensor is able to capture more information from each snap than a smaller sensor camera. The sensor on a 5DmkII is roughly 40% larger than that of a crop (APS-C) sensor. The other effect of having a larger sensor is the natural ability to achieve a shallower depth of field than a smaller sensor. See the chart below for a comparison.