'Resonance' & 'Delocalization of Electrons'

Generally speaking, there isn't any difference between the terms 'resonance' and 'delocalisation of electrons'. One would probably say this thought (eg. bezene ring):
'The resonance structures of benzene rings are due to the delocalisation of electrons between the double bonds.'
However, when one starts looking things like enols and ketos, the delocalisation of electrons in these structures would lead to what's called 'tautomerism' - where a process of proton transfer occurs. This is still due to the 'delocalisation of electrons' but different to the case of benzene (also a difference of arrows to be used!)
Planar structure can be one of the requirements for resonance to occur BUT more importantly, resonance occurs mainly due to how close are the orbitals to one another and whether the orbitals are ready to share electrons.
Sulphate isn't planar but it has a few resonance structures as you could probably draw yourself. This is, of course, due to the -ve charges on the oxgyens, which can be delocalised.
Hope that helps.