video-frequency-troubles

i still do not understand the relation between the frequency in windows and in the monitor.
when i change to a monitor which has lower frequency is the adapting-precedure the same as if its vice versa(windows has lower resolution than monitor. i always have to go in with f8(safe mode) and make the changes. do i have to increase windows-freqency to the monitor or bring the monitors higher freq. down? or can i decide?