In science and engineering, conductors are materials that contain movable charges of electricity. When an electric potential difference is impressed at separate points on a conductor, an electric current appears in accordance with Ohm's law. While many conductors are metallic, there are many non-metallic conductors as well. See electrical conduction for more information on the physical mechanism for charge flow in materials.
Under normal conditions, all materials offer some resistance to flowing charges, which generates heat. The motion of charges also creates an electromagnetic field around the conductor that exerts a mechanical force on the conductor. Consequently, a conductor of a given material and volume (length x cross-sectional area) has a limit to the current it can carry without being destroyed thermally or mechanically. This effect is especially critical in printed circuits, where conductors are relatively small. See design guidelines: http://www.eese.bee.qut.edu.au/students/pcb/Conductor_Thickness_and_Width.jpg (graph).
A special case of a conductor is the superconductor, a conductor that does not offer any resistance to its flowing electrons at all. Unfortunately, superconductors do not work at room temperature - the best superconductors today have a critical temperature of around 138K, or about halfway between absolute zero and room temperature.
See conductor article for other meanings of conductor.