Join flickr and read the discussions, for which there are many, like this one, and this other one, and the discussions have been continuing even after the cows have come home ...
One is compressed in Jpeg format straight in camera (using the camera's programming) and the other is the data of the photo as it is detected from the camera's sensor. (some Raw are compressed as well)
thanks . one more question, i using the 400d package software to convert the format, should i do all the colour correction in ps after convert or before using the cannon software ???