What's the Deal with Bulletproof Windows?

I've been thinking, what is it that stops a bullet from going through a bulletproof window? Does a force come from the window to reflect/stop the bullet? The way I figure it is that whatever material the window is made of is strong enough to repel the force of a bullet, but this must mean that somehow, the window is returning a force greater than that of the bullet, right? I'm seriously asking because I can't figure this one out and once again, Google is too much effort for me. Someone comment and confirm my hypothesis, I don't think this post will get me credit but I do still need to know.