When current flows through a wire it generates an electromagnetic field, which can then induce a current flow in other wires. Wires that carries a signal will often be shielded to prevent EMI from corrupting the signal. The shielding will reflect and absorb EMI, transmitting it to ground. Shielding is not always required, but should be strongly considered in environments such as crowded labs or factories that contain many cables, power lines, electric motors and other things that may generate EMI. It is important to note that a shield must be well grounded in order to work effectively. Source:  http://www.measurement-testing.com/EMI-twisted-pair.html