Too long to answer, check out www.bicycletutor.com, www.parktool.com or www.sheldonbrown.com. These sites will tell you all about it.
The gears on the rear wheel are called a cassette
lever at crank, connected by chain to lever at rear wheel
The rear wheel has a different hub; the gears attach to it. Alternatively the rear hub has gears or a brake or a dynamo built into it. The front hub will sometimes have a dynamo built into it, other than that it is just a plain hub.
That bicycle has 18 gears.
an average bicycle has up to 24 gears but a bicycle has up to 30 gears
This question is too wide and vague to be properly answered. At best guess the answer is NO. Gears comes in several different combos and they don't all intermix.
There has to be a gear cluster attached to the rear wheel , this is connected to a lever on the handle bars, allowing you to move the chain to each of these gears to give different speeds. Usually between 3-6 gears on this.
A tutorial on how to adjust bicycle front cantilever brakes can be found on Youtube in the form of a video outlining the key steps one should take to adjust the bicycle front cantilever brakes.
First gear on a bicycle with external/derailer gears is with the chain on the smallest chainwheel(by the pedals) and the biggest sprocket(by the rear wheel).
you have to buy and install adjustable cam gears. but if you adjust it improperly you will lose power/gas mileage.
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Yes, it's possible to prevent a bicycle from shifting. Easiest would be just not to move the shifter.