In general, metals make good electrical conductors while nonmetallic solids are good insulators, or materials that don't conduct electricity well. Electrolyte solutions, such as salt water, are also good conductors of electricity.
An electric current is the flow of an electric charge as an electron passes from atom to atom. In order to pass electrons, a substance must have space for electrons to move in while holding electrons loosely enough that they pass on easily. Metals have outer valence-shell electrons that wander, moving freely from atom to atom. When a new electron is introduced, it causes a chain reaction, much like a row of falling dominoes, that pushes electrons on down the line, carrying the current. Dissolved substances with an ionic bond, for instance table salt dissolved in water, carry charges similarly, but by passing electrons through ions instead of in the outer valence shells.
Pure water does not conduct electricity, as it contains no ions. In order for water to be an effective conductor of electricity, it must have ions dissolved in it, such as salt or small amounts of metal. Water itself has a neutral charge and, unlike metals, does not have space in its atoms' valence structures to hold extra electrons.