Every cyclist has an ideal "cadence" (pedalling speed), and an ideal amount of resistance from the pedals. When you are pedalling at your ideal cadence, you are putting out the greatest amount of power that you are able to sustain efficiently. You select your cadence by shifting gears. The gear needed to allow your "ideal" cadence will depend on the slope of the road, the wind conditions, and your own condition at any given time. Good cycle computers will give you a cadence display which is very helpful in training yourself to spin the correct cadence.

High or Low?
"Higher" gears put more resistance on the pedals. If you select a gear that is too high for the conditions, it will force you into a slower cadence. Pedalling slower than your ideal cadence is wasteful of energy. You also run a higher risk of muscle strains and joint damage, particularly to the knees and hips. "Lower" gears make the pedals easy to turn, so it becomes easier to spin to a fast cadence. Pedalling faster than your ideal cadence can allow you to generate an extra burst of speed, but you will tire yourself out too soon if you try to maintain an excessively fast cadence.

"Pushing" vs. "Spinning"
"Pushing" a high gear at a slow cadence (50-65rpm) is like power lifting. It is good for building up muscle mass and bulking up your legs, but it does little for your heart or lungs, and you can hurt yourself if you over do it. This should be only done in interval training as a strength building exercise.

"Spinning" a lower gear at a rapid cadence (90+rpm) is more like swimming. The rapid motion, with many repetitions makes the legs supple and flexible, it is highly aerobic, and the light pressure that goes with this style reduces wear and tear on the joints. With practice "spinning" becomes easier and more comfortable.