In some of the materials, electrons move from atom to atom easily. In others, the electrons move with difficulty from atom to atom. And in some of the materials, it is nearly impossible to get them to move. An electrical conductor is a substance in which electrons are in motion.
The best conductor is pure elemental silver at room temperature. Copper and aluminum are excellent conductors of electricity. Iron, steel, and various other metals are fair to good conductors of electricity.
In most electrical circuits and systems, copper or aluminum wire is used. Silver is not practical because of its high cost. Some of the liquids are good electrical conductors for instance Mercury. The salt water is also a fair conductor.
Gases are, generally, poor conductors of electricity. This is because the molecules or atoms are generally too far apart to allow a free exchange of electrons. But if a gas becomes ionized, it is becomes conductor of electricity.
Electrons in a conductor don't move in a steady stream, like molecules of water through a garden hose. In its place, they are passed from one atom to another right next to it. This happens to innumerable atoms all the time. As a result, literally trillions of electrons pass a given point each 2nd in a typical electrical circuit.
You might imagine a long line of people, each 1 passing a ball to the neighbor on the right constantly. If there is abundance of balls all along the line, and if everyone keeps passing balls along as they come, the result will be a stable stream of balls moving along the line. This represents a good conductor.
If the people become tired, and do not feel like passing the balls along, the rate of flow will decrease. The conductor is no longer good.