You seem to be confusing (as I did) ground with protective earth. "Ground" is the reference point for the voltage potentials in the circuit, is designated 0V, and all other voltages are compared to that. Consider this circuit, where power is provided from two 9V DC batteries in series:
simulate this circuit – Schematic created using CircuitLab
Any of the points A, B, and C may be considered ground but the circuit will be considered to have different voltages depending on which point is selected. If point A is ground, B is -9V, and C is -18V; if B is ground, A is 9V, and B is -9V; if C is ground, A is 18V, and B is 9V.
Protective earth, on the other hand, is used in high-voltage AC circuits to provide a safe path for unexpected current, protecting people and property from electrocution, failure, and fire.
Unfortunately, both "ground" and "protective earth" are often referred to simply as "ground" (especially, in my experience, by people who work exclusively with one or the other but not both).
Caveat: I'm new at this too, and hoping to learn from this answer as well. If there's anything I've missed (or I'm just plain wrong), please don't hesitate to comment and/or edit! Thanks.