Can someone explain to me (ms time) with monitors.

Im not so up to date with monitors, as Ive stuck with an hd 60hz aoc one for a while and things were fine on my older hd 5850. I now hace a hd 7950 and notice more screen tearing due to the higher frame rates thanks to the card and i know vsync helps but id rather not use it. Im looking at higher hz monitors but i am a noob when it comes to them. What is ms. Is it a response time of the monitor and has it anything to do with the screen hz. Also can i get away with a cheaper 100hz or will it have to be 120hz.