Resistance, or impedance, decreases electrical current per Ohm's law. Ohm's law gives current as a ratio of voltage to resistance: 1 volt flowing through 1 ohm of resistance produces 1 ampere of current. Resistance is the property of an object, and resistivity is the property of the substance of which the object is made.
Resistivity varies between materials. Copper, silver and aluminum have very low electrical resistivity; therefore, they make excellent conductors. Salt water also makes a good conductor. Air, rubber, glass, stone and ceramic have very high resistivity and thus make good insulators.
When current is passed through a wire, the negative charges repel free electrons through the wire from one atom to the next until the reaction reaches the other end of the wire. Electrical resistance occurs when substances have few or no electrons that may travel. Heat also affects electrical resistance. In most metals, more heat means more resistance because the molecules vibrate more strongly and collide, taking energy from the electrons.
In wire, thicker wires carry more current because there is more space for electrons to travel without rubbing against one another, creating excess heat and potentially starting a fire. Thicker wires are used for high-power applications for this reason.