For questions relating to the neutral wire, or "grounded conductor" as it's sometimes called. The neutral, or "grounded conductor" in NEC, is the normal return path for electricity in a household AC circuit.
The neutral wire provides a return path for electricity from a utilization device back to the main power transformer (and thus, the rest of the electrical distribution system). The NEC calls it a "grounded conductor", while other folks may call it a "return".
In most systems, the neutral wire is bonded to the safety grounding system at the main disconnect or service entrance -- sometimes, this bond takes place in the meter base. This bond is what allows the grounding system to serve its safety function of causing a breaker to trip upon a hard fault from hot to ground. This does not mean that the neutral is safe to touch, though -- it is a live conductor, and will still bite! (It will also have a small voltage on it relative to the mains earth due to voltage drops/losses in the neutral conductor.) Furthermore, cross-connecting ground to neutral at other points within the wiring system is not allowed -- such a practice energizes and defeats the safety ground system due to parallel current flows.
In North American convention, white or grey wires, or white wires with colored bands/stripes, are used for neutrals as per NEC article 200. Other countries may use black (old UK) or blue (IEC) wires for the neutral conductor. Due to the aforementioned bonding requirement, however, no matter its color, the neutral shall not be switched without switching the accompanying live wire(s) -- this requirement is needed to make old Edison-base lampholders reasonably shock-safe, and also prevents a grounded neutral fault (i.e. a cross-connection between ground and neutral at an inappropriate point) from inadvertently bypassing the switch.