The link Darkstar posted gave an error for me, so I don't know what's on that site. Here is my short explanation:
Resonance structures are different Lewis dot structures of the same compound. These isn't always more than one Lewis structure that is reasonable, but often when you have a conjugated double bond or charged atom in one Lewis structure, there will be another Lewis structure that is related by resonance. Resonance structures are related to one another in that they can be drawn by pushing a pair of electrons, usually to form a double bond, while displacing electrons from another double bond. Resonance structures can never break a sigma bond, and resonance structures can never rearrange any atoms. Resonance theory is really an approximation of what is really happening in a molecule or ion. The resonance structures represent the most extreme posibilities for bonding and charge separation that are reasonably possible. The actual molecule or ion is a mixture of *all* the reasonable resonance structures, with somewhat of a statistical distribution based on how "good" each resonance structure is.
Check out a couple of examples (ozone and nitrate) on this site: http://wine1.sb.fsu.edu/chm1045/notes/Bonding/Resonan/Bond07.htm
Hope this helps.