I agree with 99jon's post. However, I have never heard of this"layer" concept.
Lightroom stores everything you do, within the application, in the Catalog File. Lightroom builds Previews at import which allows you to browse in the Library module.
While you work in the Develop Module, Lightroom works directly with the Original File and reads from the Catalog file, re-rendering as you work. This process is assisted by the Camera Raw Cache files. The order that is used by Lightroom in applying the edits is decided by Lightroom and applied when the file needs to be re-rendered. This is why there is a delay in display in this module when lots of edits have been applied.
Hi DdeGannes and thanks for your reply, Concerning "layers" maybe I explained myself wrong. I know Lr store any edit in the catalog and I know that rendering in develop module is made almost in real time. What I wanted to say is that, in the end, Lightroom in his own order of applying edits keeps implicitly the concept of "layer". If I open a raw file and then I apply a profile, I can see "as if" the profile is on top of my photo. To be "on top" recall me the concept of layering. If, after applying the profile, I take the WB picker and I click on any point of the photo rendered on the screen, the WB picker doesn't work on the RGB values shown by his own loupe, but sampling RGB values from the "underlying level" that is the actual photo. In summary, what I wanted to say is that, even if we can't really talk of layers, I find the concept of layering is still present. Regards.
P.S. Also think to the profile opacity slider: opacity is one of the main traits of layers.