How much electricity in volts does the iPod touch second generation's USB cable pull from the computer?

All USB peripherals are supplied at 5.0v.

For v1.1 and v2.0 USB connectors there are 4 wires:

5v power feed limited to 0.5Amps

Ground (think of this as the -ve side of a battery)

Data +

Data -

These data wires are what does the actual data transfer.