Well it depends where you live.
In Australia it's a Night VFR rating. To qualify you have to do a cross-country flight test at night and -- Australia being what it is -- that usually means flying over plenty of dark landscapes.
You also have to demonstrate proficiency with navaids (for track keeping) and my NVFR is endorsed NDB and VOR. There was no GPS in those days.
No doubt some of the requirements may have changed since I did mine many years ago. At the moment I'm not current for night flying but I used to find it most enjoyable.
I believe that once upon a time a NVFR rating was known as a Class 4 instrument rating. I have no idea what was meant by Class 1, 2, 3....