All conductors lose a certain amount of signal over distance. The "bigger" a conductor is, the less signal is lost over distance (you'll see why this is important later).
Also, dual/quad shield refers to how many layers of "shielding", or layers of "silver metal", cover the dielectric.
The dielectric is next, which is the opaque, white, waxy part of the cable. This is insulation from shielding.
The shielding is the layer of braided wire. The purpose of the shielding is just that. It shields the conductor from stray "outside" signals, also known as ingress (more on ingress later). It also keeps the signal being carried along the conductor INSIDE the cable (prevents egress, which is signal leaking OUT). Most RG6 has 2 layers of shielding (dual), but you can also find quad-shield (4 layers) cable out there. It is more expensive, and harder to work with.
The last layer, of course, is the rubber. This is just more insulation and there's no point in trying to describe what IT does.
Regular Coax Cable:
Dual-Shield Coax Cable:
Quad-Shield Coax Cable:
These pictures and more information on different types of coax can be found at:
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