Shortest, easiest way for me to explain is: Larger diameter requires more power to spin and moves larger volume of air, smaller diameter spins quicker and at higher RPM using less power. The pitch can be thought of as how far forward the prop moves with each revolution. A lower pitch prop will provide more torque to instantly yank your plane into the sky, a higher pitch will produce a higher top speed, but it takes longer to build up that speed, especially on take off. You have to find the perfect middle ground between that instant torque and speed based on your engine and plane. Sometimes experimenting is the only way to find it.