How does an electrical ground work?

Okay I am trying to learn how electricity works.

So from what I have read you first have a battery which contain electrons. When you connect a wire to the negative side of the battery and then connect that to a light bulb, and then connect the light bulb to the positive terminal via another wire, the bulb will light up. Meaning the electrons will flow from the negative side of the battery to the positive side, and because the bulb is in the middle of that flow it will light up.

Well what I don't understand is what a ground wire does? When I am working on my little motorbike some things are grounded to the frame. If the wires eventually end up at the positive terminal, why would something need to be grounded to the frame?

When something is being grounded what is taking place? I also looked at a diagram of my bike and it looks like the electron flow is no longer going from the negative to the positive but now from the positive to who knows where. http://www.sdscooters.com/Tech/Super_Pocket_Bike_Wiring.html

Can anyone explain these mysteries to me?

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electrician brighton|plumbing haywards heath