Electric wires are made of a conductive metal, often copper, worked in thin wires to form a single.
This single cable is called a soul. Depending on the number of souls that form the cable can be unipolar, bipolar, tripolar, quadripole, etc.
Each cable must be insulated by a sheath often made of PVC plastic material.
There are special cables for special applications, such as high temperature resistant electrical cables with insulation made of silicon material.
Extensions, though very common, are to be used with caution to avoid overloads and must always have contacts for grounding.
Slip-on extensions have plugs that do not have to be overcharged, take note of the technical specifications reported to prevent accidents.