Solids and liquids that conduct electricity include metals and water. These materials are called conductors because they allow electrical current to flow through them.
Materials can be defined by their electrical conductivity. Materials which do not conduct any electrical current at all are called insulators. There is no such thing as a perfect insulator, but materials which resist current the most strongly include glass, paper, ceramics, most plastics and generally most non-metals. Materials which can conduct some current, but don’t do it very well, are called semiconductors. These include silicon and germanium, and are used frequently in electronics.
Materials which conduct current well are called conductors. Solid conductors include most metals, such as copper. This is why electrical wiring is made of metal, with plastic coating on the outside; the electrical current flows down the metal wire but is insulated by the plastic coating. Gold and silver are also very good conductors.
For a liquid to conduct electricity, it must contain a high concentration of ions. Distilled water is a relatively poor conductor because it has very few ions. Adding ionic solids (such as table salt) to a liquid will increase its conductivity. Tap water is generally a very good conductor because it has plenty of ions. So is lake water, which is why it is dangerous to swim during a lightning storm. Water which contains an acid or a base also makes a good conductor, because when an acid or a base is dissolved in water it breaks down into ions. Sulfuric or hydrochloric acid dissolved in water make good conductors. Vinegar is generally too weak an acid to have the same effect.