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At the announcement he mentioned that there would be multiple availability zones - which we could treat as yeffectivly different data centers. I thought they were _literally_ different datacenters. What exactly is an availability zone?

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There is physical separation between availability zones, though I don't think it's been made clear what that separation entails on a brick-and-mortar level. Here's Amazon's description: "Availability Zones are distinct locations within a Region that are engineered to be isolated from failures in other Availability Zones and provide inexpensive, low latency network connectivity to other Availability Zones in the same Region."

I've always envisioned AZs in a given region as separate facilities on the same fiber loop.

Historically, we know that power and network outages have generally been isolated to a single AZ, supporting their claim of a high degree of isolation.

Here's the relevant page in the EC2 docs: http://docs.amazonwebservices.com/AWSEC2/latest/UserGuide/in...

It may or may not be different data centers, I think they want to abstract that concern away from you. The purpose of an availability zone becomes more important from a billing perspective. Data transferred within a zone is free but if you go between zones you start incurring bandwidth charges.

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