See Screening

The terms screening or shielding normally refer to a method of preventing external electromagnetic interference reaching sensitive electronic circuitry by encasing it within an electrically conductive enclosure, often referred to as a Faraday Cage.

The shielding works by distributing the electrical charges induced by an external electrical field in such a way their influence is cancelled out in the interior.

Often, the shielding enclosure is connected to ground, to drain away any induced currents, although this isn't strictly necessary.

Most electronic equipment is housed within a metal chassis, and most cables have an outer shield or screening layer in their construction, specifically to minimise the unwanted interference from external fields. 

This SOS article discusses the many variations of cable shielding technologies.

See Screening