Metals are able to conduct electricity because they have valence electrons. These are electrons that can move through the atoms that make up the metal. As they do so, they are able to pass along electric currents as they repel other electrons in the atom.
All materials can conduct electricity, but non-metals, such as glass, cling to their electrons and don't allow them to move around much, making whatever current they carry negligible. These materials are considered insulators.
Electrons have a negative charge, attracting them to matter that has a positive charge. An applied electric field causes the electrons to move by the electromotive force. Though electric waves move at speeds close to the speed of light, electrons are slowed down as they pass through the conductor. This is called electrical resistance, and the greater the conductor's resistance, the smaller the current that can pass through it. This resistance is measured in ohms, while the electromotive force is measured in volts.
Metals, such as silver and copper, are especially good conductors of electricity because they have only one valence electron. Silver is an even better conductor of electricity than copper, but is too expensive to use in most electrical systems. Gold and aluminum have are also excellent conductors of electricity.