First, technically, there is no neutral in a single phase system. The white (or natural grey) wire is called the "grounded conductor". This is not to be confused with the "ground" (green or bare wire) which is called the "equipment grounding conductor".
In a 120/240 Volt Single Phase system, the transformer secondary coil is "center tapped" to ground which defines the grounding conductor's voltage as "zero". The right and left extremes of the secondary coil are the taps for each 120 Volt leg. Voltage from either the right or left tap to the grounded center tap will be 120 Volts or thereabouts. But the voltage between the left and right taps are 240 Volts, not involving the grounded conductor at all.
Some circuits are considered 120/240 Volt. Your Dryer, Range, and the service for your house fit this description. These contain a grounded conductor as well as two hot conductors. This is to provide a grounded conductor for motors, timers and buzzers that operate at 120 Volts. Usually the heating element is the only 240 V draw on a dryer. Water heaters only require a "straight" 240 Volt circuit.
All circuits require equipment grounding conductors! (In residential, anyway...)
I'm kind of going out on a limb here...
Correct me if I'm wrong guys...
The current flows towards the load in both wires regardless of the type of circuit, straight 240V or a 120V to ground. The voltage alternates at 60 hertz, but the current flow still goes in one direction.
Okay, what about DC? The voltage may be - and + but the current still flows from both wires towards the load. The voltage gives the impression that the flow is circular, like a chain on a bike. This is not the case from what I am to understand.
I hope this makes it all clear as mud.
Please get yourself a copy of the National Electrical Code 1999 NFPA 70 if your serious in dabbling with electricity. It is not a "how to" manual, but you should make yourself familiar with it.
Other sources of info are here in the Electrical Contractor Network, and you can visit my site at http://www.kellyelectric.electrical-contractor.net
for some tips for the homeowner.
Good luck and feel free to post questions anytime. If you have any doubts about something, don't hesitate to ask one of us here!