with JPG it's a lossy compressed format - so everytime you save a jpg you throw away a little more detail
the psp format, photoshop, bmp & tiff are uncompressed and so most pros use these formats to process their pictures before saving the finished pic to JPG.
however all these end products have already been processed in the camera from it's raw data.
different versions of firmware can make very different decisions on what adjustments to make
.... it's a bit like preset white balance - you set for fluorescent - but the fluorescent is a daylight and not a white
and so the pic comes out a bit red - you then use psp to reduce the red and return the balance
all fine - but the picture has then been processed twice
to alter the temperature of the pic + whatever else you want
and thus compressed twice ... and so will have dropped something like 10% of the original info minimum
raw is unprocessed and uncompressed - you make all the adjustments you want and save - 0 loss.
generally if there is ant doubt pros will slightly under expose - an under exposed pixel can generally be lightened whereas a burned pixel is just white ....
using a raw processor you can often rescue a total disaster
in JPG the info has already gone because the camera has already "optimised" the exposure
psp is good - I use it at home myself - the wife even has a go .... but photoshop is better - I use that at work - on raw files
http://en.wikipedia.org/wiki/RAW_image_format