Region vs. Availability Zone: what the difference actually means
Kort antwoord
A region is a geographic area a provider operates in. An availability zone (AZ) is one physically distinct facility within that region, with its own power and network, close enough to other AZs in the region for low-latency links between them but far enough to fail independently. Picking a region decides roughly where your data lives. Spreading across AZs decides whether a single facility problem can take your whole setup down with it.
Region: where, in broad terms
A region is the coarse-grained choice: a country or metro area a provider offers infrastructure in. Choosing a region mainly answers two questions: how close is this to the users or systems that will talk to it, and which legal jurisdiction does the data sit in. Neither of those questions is about resilience. A region on its own says nothing about how many physical buildings your infrastructure could be spread across, or whether it's spread across more than one at all.
Availability zone: which building, and does it stand alone
An availability zone is a specific, physically isolated facility (or a cleanly separated part of one) inside a region. The word that matters is isolated: an AZ is meant to have its own power feed, its own cooling, and its own network path, so that a fault in one, a power failure, a cooling failure, a fire, a local network incident, doesn't automatically spread to the others. AZs within the same region are typically built close enough together that the network link between them stays fast, which is what makes it practical to run one workload split across two or three AZs without paying a heavy latency penalty for it.
Why this distinction matters practically
A deployment that lives entirely in one facility, no matter how well-run that facility is, has a single point of physical failure. If that building loses power, loses cooling, or has any incident that takes it offline, everything inside it goes down at the same time, regardless of how much redundancy you built at the server or application level. Redundant power supplies and RAID arrays protect you against a component failing inside that building. They do nothing if the building itself goes dark.
Spreading resources across multiple AZs in the same region is what protects against that class of failure. It means a facility-level incident takes out one AZ's worth of capacity, not all of it, and a properly designed application can keep serving traffic from whatever's left. This is a different, additional layer of protection from spreading resources across multiple servers or racks within a single facility: server-level redundancy protects against hardware failing, AZ-level redundancy protects against the facility itself failing.