Simply put: VFR, Visual Flight Rules, you can see outside, you must stay outside clouds, it's your job to not hit other planes and the ground. You do this primarily by looking out the window.

IFR, Instrument Flight Rules, you are in the clouds and can't see outside, it's air traffic control's job to keep you away from other airplanes and the ground. You keep the plane under control by using your instruments only. Looking out the window yields no information. :-)