Resonance structures are multiple ways of drawing the electron distribution of a single, unchanging molecule — the atoms never move, only the electrons (specifically pi electrons and lone pairs) are redrawn in different positions across the different resonance forms. Because nothing about atomic position changes, resonance structures aren't separate compounds at all; they're contributing pictures of one real, single structure that's actually a blend (a 'hybrid') of all the resonance forms drawn.

Tautomers, by contrast, are genuinely different compounds — the atoms themselves rearrange, usually with a hydrogen physically migrating from one atom to another, along with a corresponding shift of a double bond. Because atoms actually move, tautomers are, at least in principle, separable, isolable compounds that can rapidly interconvert through an equilibrium, rather than being two drawings of one single unchanging structure.

The most common tautomeric pair you'll encounter is the keto form and the enol form of a carbonyl compound. In the keto form, a hydrogen sits on the carbon alpha to the carbonyl, and the carbonyl itself is a normal C=O. In the enol form, that same hydrogen has physically migrated onto the carbonyl oxygen instead, and a new C=C double bond has formed between the former carbonyl carbon and the alpha carbon it came from — genuinely different bond connectivity, not just a different electron-pushing picture of the same connectivity.

For most simple ketones and aldehydes, the keto form is overwhelmingly favored at equilibrium (often well over 99.9% keto), but the enol form — even though it's a minor, fleeting player most of the time — turns out to be mechanistically essential, since it's the actual nucleophile in reactions like the aldol condensation and alpha-halogenation of carbonyls that you'll encounter later in this course.