Resonance structures result from electrons not being fixed in position (that's why you "push" electrons when drawing resonance structures).
When electrons are not fixed in position, they are delocalized electrons.
For all practical purposes, resonance and electron delocalization mean the same thing.
In ions, resonance and electron delocalization occurs to "distribute" the charge around.
In molecules, resonance and electron delocalization occurs in aromatic rings and conjugated double bonds.