hi,
CNS (i'm thinking you don't mean central nervous system)
Clincal Nurse specialist
Masters prepared Registered nurse with a focus on a specific domain of patient population, i.e., geriatrics, critical care, pedi, oncology (you get the idea).
Psychiatric CNS's can also prescribe (and i think do some sort of therapy/counseling).
They function is many hospitals as the overall 'experts' in those areas and are fundemental to education and progressive care in hospitals. This is a brief description. I'm sure a CNS could go into more depth.