Live and neutral
As far as the electrons travelling through the wires are concerned, there's no real difference between the two wires.
In real life, one of those is connected to the ground and called 'neutral'. As long as you've got your feet on the ground (or thereabout), you're indirectly connected to it. That's why touching the other ('live') wire hurts - you're now connected to both.
Technically it would be safe to touch the 'live' wire while in free fall or otherwise disconnected from the 'neutral' wire. You still shouldn't try it though.
(This would be the same with DC, only you'd need to consider polarity instead of the phase.)
Biting the hand that feeds IT
