Difference between ICR1 and OCR1A - in Fast PWM?

I am not trying to measure a pulse length. The code does that(I pulled it out from the forum here). The duration of the square wave peak to peak is 8us. I meant input capture. I am not so clear with the terminology - apologies. Putting that aside, could you show me how to generate a pwm waveform? I am not sure what mode to choose. Maybe once I understand that, I can ask you any further questions.

Best,
A