It's not always at 0V.
When you measure voltage, there must be some reference point, so if your reference point is ground, then you are measuring the same points electrically since ground and neutral are tied together.
Neutral is just the other side of a transformer winding. In the case of 240v systems, it's the center tap of that transformer so that each "hot" to neutral is 120V, and hot-to-hot is 240v.
http://www.hsdivers.com/Ham/images/xform...
Here you see the neutral tied to ground:
http://forum.allaboutcircuits.com/image_...
If the Neutral was lifted from ground, there would be no difference between hot and neutral, electrically. They are just different sides of a transformer secondary winding.
It's not always at 0V
Current flows down the live wire, through the load, and back up the neutral wire to the source (in a house, that's probably some transformer on the street).
The wire has low resistance, but not zero. So suppose it's 120V at the transformer, it's 119V at the live side of the load, 1V at the neutral side of the load, and 0V at the neutral side of the transformer (whihc is grounded).
"Neutral".
Current can flow at any potential. In this case, the neutral wire is the return for the current from the hot line. It's at zero volts (nominally) because the neutral is tied to ground at one point.
You could just as easily (not recommended) tie the hot to ground instead, and things would work the same way, with the same current, except that the neutral would now be at a potential of 120 VAC and the hot wire at zero volts.
Ideally, the voltage drop across a wire is 0 volts; this is nothing new. The common or neutral connection is generally at 0 volts, but there is a current in it.
How does the neautral wire conduct electricity if its always at 0v
