The stage of a melanoma is a description of how widespread it is. This includes its thickness, whether it has spread to the lymph nodes or any other organs, and certain other factors. The stage is based on the results of the physical exam, imaging tests (CT or MRI scan, etc.), biopsies, and other tests, which are described in the section, “How is melanoma skin cancer diagnosed?”
Here is an indepth discussion of melanoma staging.
If you want a more graphic resource see: the National Cancer Institute Site.