What is the difference between a high gear and a low gear on a bicycle?What is the advantage of each gear?

Answer 1

That depends on what you mean by high or low gear.
Most multi-speed bikes have rear gear assemblies and front gear assemblies.
On the front gear if the chain is on the small gear. The amount the chain moves in a complete pedal rotation is small. The larger the gear the more the chain will travel given a single rotation.
So for the from the larger the gear the faster you will go.

The back gears work in reverse. Here it is not the gears driving the chain, but the chain driving the gears. A small rear gear takes less chain to spin it all the way around than a large gear. So given the same length of chain the smaller gear will rotate more. The increased number of rotations mean that you are moving farther given each full pedal rotation.

So the fastest gear combination or high gear, would be the largest front cog and the smallest back cog.

The trade off to speed is it is much more difficult to pedal in high gear because you are generating much more momentum in that pedal stroke.
A low gear which only moves you a little per stroke means that the pedaling is much easier.
Low gears is used when starting from a stop or going up inclines. You generally switch to higher gears to increase your speed without having to pedal faster.

Answer 2

High gear gives you a higher ratio of revolutions at the wheel as compared to the revolutions of the pedals. Low gear is the opposite, less revolutions of the wheel as compared to revolutions of the pedals. It's based on the number of teeth on each sprocket. In high gear you may have 100 teeth on your pedal sprocket, and 50 on the wheel sprocket. So for every full revolution of the pedals, you would turn your wheel twice. In low if you had less teeth on the drive sprocket, say 50 for example, it would be a 1 wheel turn for every turn of the pedals. In low gear you would go slower, but with less effort... handy when riding up-hill. Most 10 speeds have 2 sprockets in the front, and 5 in the rear. You lowest gear combo being the small front sprocket and the largest rear sprocket. The highest gear ratio is achieved with the largest front sprocket and the smallest in the rear.