The main difference is what has to happen before the zone. A supply or demand zone is drawn on the strength of a move alone: there was a consolidation, price left it sharply, mark it. An order block asks for more: the move must not merely be strong, it must break market structure.
Outwardly both are a rectangle where price departed from. The difference is what earns the right to draw it.
What that changes in practice
Supply and demand zones are plentiful. Sharp exits from consolidation occur on any instrument, and the markup comes out dense. That is both a strength and a weakness: there is plenty to choose from, and the choosing falls to you.
An order block requires context. Until a previous significant level breaks, there is no block, however sharp the move. So blocks are noticeably rarer, and each is tied to a specific structural event.
The zone's boundaries
The approaches differ here too.
A demand zone is usually drawn across the whole consolidation — top to bottom, sometimes by bodies, sometimes by wicks, with no single rule: as many variants as there are sources.
An order block is defined more precisely: the range of one specific candle, the last one against the direction of the move before the impulse. The only thing left to argue about is whether to take the body or the body with wicks.
When they coincide
Often. An impulse that breaks structure usually leaves some accumulation behind, and the last candle before it lies inside that accumulation. The block then sits inside the demand zone — a smaller part of it, in the same place.
They part company in two cases. A strong move without a break of structure gives a demand zone but no block. And the reverse: a break can follow a gradual approach with no clear consolidation — there is a block, but no demand zone as such.
Which is better
An unanswerable question: they are two ways of marking the same observation — "price left here sharply". An order block is stricter and rarer, a demand zone is broader and more frequent.
Strictness does not make markup truer. It only means the cut-off is explicit and the same in every case, rather than left to whoever is drawing.
What neither of them gives you
Neither approach claims price will return to the zone and bounce from it. Both mark the place a move started from — a coordinate on a map of the past.