A CNS is a masters-prepared RN. Here is a nice concise idea of what they do: http://www.ga.unc.edu/NCCN/recruitme.../roles/cns.htm
I am a CNS in IL and I am considered an advanced practice nurse. In IL, the nurse practice act is the same for NP, CNM, CRNA and CNS. I have prescriptive authority, my own NTI number (for billing Medicare for my services) and actually am quite autonomous. I work for a large nephrology practice and round on chronic hemodialysis patients. I work with 15 MDs, a total of 8 midlevels and me!