Q: What is a Shield?
A: A shield is an electrical barrier such as an aluminum/mylar tape or served wire that resides beneath the jacket of a cable or device. It provides both an electrical barrier as well as a mechanical benefit of strength and cable protection. The shield in the case of a coax is the inner layer that sits above the dielectric enabling the capacitive effect. It is traditionally a metalized PET tape or spiral served wire.
Shielding effectiveness is very complex to measure and depends on the data frequency within the cable, the shield design and the means in which the cable is used and abused. A shield may be very effective in one frequency range, but horrible in another. If certain design factors are not taken into account or the cable is used improperly then shielding will have an impact on the overall success of a design.