Both spread your VMs so one failure doesn't take the whole application down — but a set spreads them across racks inside one building, and a zone spreads them across separate buildings, and that difference is the whole point. Two identical web servers behind a load balancer buys you nothing if Azure puts them on the same rack and that rack loses power. Availability sets and availability zones both exist to stop that — they take your VM instances and pull them apart onto independent hardware s...