What's the key difference between conjugated system vs resonance?

Question

They're used interchangeably at times, but they're supposed to have a key difference.

Answer 1

Conjugation specifically describes any molecule or ion with three overlapping p orbitals in a row, such as allyl cation, anion or radical. This is a special thing that lowers the energy of the molecular orbitals and stabilizes the molecule. A more strict definition of conjugation is limited to polyenes, such as conjugated dienes, that have four or more overlapping p orbitals. Conjugated polyenes are more stable than isolated polyenes.

Resonance is a more broad term used to describe situations where we can draw multiple lewis structures for a molecule by only changing the position of electrons. Usually, we limit this to pi electrons but technically you can also draw resonance structures where sigma bonds are broken ("no bond resonance").

A ketone is an example of a common functional group that has a resonance structure, but doesn't have conjugation.