In electronics, ground is usually just a "common" or "reference" connection. It may, or may not, be connected to earth ground.
For example, the negative side of a car battery is connected to the car body and that's called ground. If you connect a light bulb between +12V and the car body it will light-up. If you connect a light bulb between the +terminal of a car battery and earth ground, no current will flow and it will not light up. Or if you have 2 cars sitting side-by-side, a light bulb connected between +12V on one car and the body of the other car will not light-up.
It's quite common for a circuit to have both positive and negative power supplies. For example, an op-amp may operate from plus and minus 15V. You can measure +15V to ground or -15V to ground, and if you put your voltmeter between +15V and -15V, you will measure 30V.
If you simply connect a light bulb to a battery, there is no ground.
Basically, you as the engineer define a particular place as being 0V. Often, this is "Earth Ground" but like Doug said, sometimes you aren't connected to earth ground, so while calling your negative terminal on a battery "Ground" is technically wrong, but everyone understands anyways. As to how electrons flow, it is actually not all that direclty related, though it is useful for calculating voltage potentials and currents in different parts of a circuit. It is like defining your origin, but for voltages.