Simply put, an ion is molecule having a different number of electrons than protons thus giving it a negative or a positive charge respectively.
Sometimes a phenomenon occurs called ionic bonding. All except the six Noble Gases (helium, neon, argon, krypton, xenon, and radon) are subject to ionic bonding. In ionic bonding, the atoms form a crystal lattice molecule in which ions of opposite charge are bound to each other. This present a more stable configuation. Salt is just such an example of ionic bonding.
So, just like single men and women are free to roam at will, the same holds true in nature. Think of the free chlorine atoms as the singles still looking for fun, adventure, and a mate! Think of the chloride ions as marrieds with less of a tendency to flit here and there!