Resonance is a useful concept in chemistry. Earlier, we looked at the following carbocation, and explained it in terms of a spreading of charge.  An alternative (and sometimes more useful) way of descibing this phenomenon of the lowering of the energy of this species is by use of resonance.

Let us draw the molecule again, without the p orbitals or hydrogen atoms:

By moving the electrons from the double bond to the empty p orbital, we can effectively move the position of the double bond and the positive charge.  Finally, we can combine the two extreme forms into the representation at the bottom, with the charge spread over the length of the pi system.  The two diagrams at the top are known as resonance forms.

In general, the more resonance forms one can draw for a molecule, the more resonance stabilisation energy it has, and it is therefore consequently lower in energy.  However, each resonance form does not necessarily contribute the same stabilistaion energy.  In the case above, they happen to, because both forms are identical, in which case they are of equal energy.

In cases where the forms are of different energy, the main contributors are the ones of lowest energy.  For example:

Technically, we can invoke resonance to give us the slightly unusual form of ethane on the right.  Clearly, however, in this ridiculous example, the form on the right is of much higher energy than normal form on the left, as it involves the separation of charges; it therefore has negligible effect on the overall energy and structure of the molecule.

Resonance can be used to explain the surprisingly high (in relative terms) acidity of this molecule (cyclopentadiene):

If we remove H⁺ from the carbon not involved in any double bonds, we form the following species:

As you can see, it has 5 resonance forms of equal energy (hence they are all equal contributors).  This "extra" stability conferred by resonance is the reason for the increased acidity of cyclopentadiene.  In fact, as we shall see later, the conjugate base of cyclopentadiene is aromatic.