Durability

Durability is the last in the resiliency trifecta:

1. Availability
2. Reliability
3. Durability

It's about how well your data survives in the event of an outage. For example, let's say you're running your own single server:

1. What happens if the intern accidentally rm -rfs the user_pics directory?
2. What happens if the server's hard drive fails?
3. What happens if the data center it's in catches fire?

These are all durability questions. Durability is primarily about backups and redundancy. In the case of S3, it automatically replicates your data across multiple servers. If one goes down, the backups are there.

According to these docs S3's standard storage provides 99.999999999% durability and 99.99% availability of objects over a given year. Nice.