Water is known as the universal solvent because it dissolves more substances than any other liquid. It can do this because water molecules carry an electric charge strong enough to disrupt the molecular bonds of the substances it dissolves. One such substance is sodium chloride (NaCl), or table salt. When you put salt in water, the sodium and chlorine ions separate and attach themselves to individual water molecules. The resulting solution becomes an electrolyte, which means it's capable of conducting electricity.
TL;DR (Too Long; Didn't Read)
Water consists of polar molecules that attract sodium and chlorine ions. The attraction disrupts the crystal structure of sodium chlorine and produces a solution of free ions known as an electrolyte.
The Structure of the Water Molecule
Rather than being symmetrically arranged on either side of the oxygen atom, the two hydrogen atoms in a water molecule gravitate to the 10 o'clock and 2 o'clock positions, much like Mickey Mouse's ears. The asymmetry produces a polar molecule with a net positive charge on the hydrogen side and a negative one on the oxygen side. This polarity not only makes water a good solvent, it's also responsible for such phenomena as microwave heating. When microwaves pass through water, the polar molecules align with the radiation field and begin to vibrate. It's the heat generated by these vibrations that warms your food.
How Salt Dissolves
Sodium chloride is an ionic crystal. Sodium ions carry a positive charge while chlorine ions carry a negative one, and the two naturally form a lattice structure. When you put salt in water, the positive ions gravitate toward the negative sides of the water molecules while the negative ions move towards the other sides. In this way, each water molecule disrupts the lattice structure, and the result is a solution of free ions suspended in water.
Dissolution happens more quickly if you agitate the solution by stirring or shaking, because the addition of mechanical energy disperses free ions and allows more unmated water molecules to access the salt. At a certain point, the solution becomes saturated, which means that all the water molecules are attached to ions. No more salt will dissolve in a saturated solution.
A Strong Electrolyte
An electrolyte is a solution in which positive ions, called anions, and negative ions, or cations, are able to move freely. Because of this freedom of movement, an electrolyte can conduct electricity. A sodium chloride solution is a strong electrolyte because all the ions from the salt dissolve -- assuming the solution isn't saturated -- and no neutral NaCl molecules are left to weaken the conductance.
The ability of salt water to conduct electricity depends on the concentration of sodium and chlorine ions as well as on the absence of impurities. For example, while seawater can conduct electricity, seawater won't conduct electricity as well as pure saline water that has the same salt concentration because seawater contains a host of other minerals and other impurities that act as electric insulators.