Final answer:
The primary characteristic of a conductor is the presence of free electrons, which allows for the easy movement of charge through the material, differentiating it from insulators.
Step-by-step explanation:
The primary characteristic of a conductor is the presence of free electrons in its structure, which allow charges to move through the material with relative ease. Conductors, such as metals and saline solutions, enable free electrons or ions to move through them, similarly to how air moves through loose sand. This is distinct from insulators, which hold charge fixed in place. The movement of these electrons can lead to small energy losses due to collisions with fixed atoms and molecules, but the electrons are still able to move within the conductor. Superconductors, a specific type of conductor, allow the movement of charge without any loss of energy, which is a distinguishing feature.