Answer:
The structure changes in tautomers i.e. a proton leaving a carbon bond and attaching to a ketone group.
Resonance is the concept that electron density is sometimes shared between multiple bonds. In a Lewis structure, this can be expressed as double bonds swapping between two oxygens or two different set bonding set ups have Oxygen or Nitrogen bearing a formal charge.
For the actual measurement of bonds, the bond lengths between multiple resonating bonds tend to be an intermediate length between double and single bonds. This supports the idea that electron density is shared between several bonds in a resonating molecule. Benzene bonds are a good example of intermediate bond lengths due to resonance.
In Tautomerism the alpha-hydrogen is shifted where as in Resonance the PI electrons are delocalised to loss more energy and to get stability.