How do resonance structures and isomers differ?
Short answer: Resonance structures differ in the location of electrons. Isomers differ in the location of atoms.
Resonance structures represent the same compound.
For example, acetone has two resonance contributors.
They differ only in that a pair of π electrons has moved onto the oxygen atom. Only the electrons move, not the atoms.
The molecule is a resonance hybrid of the two structures.
Dimethyl ether and ethanol are isomers. They have different chemical and physical properties.
Converting dimethyl ether to ethanol requires breaking a C-O and a C-H σ bond and forming new C-C and O-H σ bonds. The atoms change positions and form two different compounds.