ok, first off personally i don't see much benefit from shooting both RAW and JPG. just shoot RAW. you'll save space over time, plus RAW is better to edit and suffers less during the process (although some features are 8-bit only, so if you really need to, convert the 16-bit RAW to 8-bit and carry on). you can then edit, save as a PSD file, then convert to 8-bit and save as a final JPG. this means you can go back to the PSD file later and you'll still have the 16-bit data and your layers will still be editable. remember, you can always convert down, but converting up will never give you data that isn't there!
layers... basically, think Photoshop as having the bottom layer of a photo (say a face), then you lay a sheet of acetate over the top of that and make some changes (say draw a mustache). to the eye, it looks like one image, but you can easily manipulate the mustache without affecting the face photo in any way (and vice-versa). you can pile on layer after layer with different changes which will not affect any other layer below or above as far as editing and effects go, though you can use each layer to change the way the other layers look.
you may find
this site useful
you can see some examples of layer work
here