1) HZ
When talking about the monitor you discuss the REFRESH RATE. In Hz. That is how many times per second it changes. Most screens are 60Hz (60x per second).
2) FPS
Frames Per Second. This is the output of the GRAPHICS CARD. If you don't cap via VSYNC or non-synced cap it will create as many FRAMES as it can until the GPU or CPU (or other) becomes overwhelmed.
3) VSYNC
This synchronizes the GPU output with the monitor refresh rate. If the GPU can keep up, AND the monitor is 100Hz then you (ideally) will have one new FRAME for every refresh. Thus always show 100FPS.
If VSYNC is ON and your GPU can't keep up the FPS indicator will just show whatever the GPU is producing (sort of... more below).
4) FPS indicator isn't always accurate...