I was intrigued by the concept of 120Hz monitors, I'm a bit behind the times.
After a bit of reading around, in essence a 60 Hz monitor can display 60 fps. If you turn vsync off the fps may go above 60 (if you have a fast gpu) but your monitor will only show 60 due to the 60 Hz refresh rate. Vsync on locks your fps to that of your refresh rate.
120 Hz obviously then increases the possible fps to 120. Seems like the majority of people swear they notice a smoothness difference with 120 Hz monitors. Whether or not everyone would is another debate. It depends monitor attributes you regard highly; resolution, refresh rate, latency, colours, viewing angles and of course cost.