I'm assuming you mean Shielded Twisted Pair, as in the type of wire.
Shielded Twisted Pair wire serves two purposes:
1) Reduces electrical interference from its own wires (this is what the twisting is for)
2) Reduces electrical interference from other wires / devices in the area (this is what the shielding is for)
Generally, as the speed of the data you transmit increases the voltage range decreases.
For instance, 10Mbps ethernet networks generally use +2 and -2 volts, while 1000Mbps networks use 1, .5, -.5 and -1 volts. The lower the signaling and line voltages are, the more susceptible to interference the line becomes.