Ask HN: What is the actual purpose of Docker?

I'm hearing about Docker every other day, but when I look into it, I don't understand the purpose of it. I run many websites/applications that need isolation from each other on a single server, but I just use the pretty-standard OpenVZ containers to deal with that (yes I know I could use KVM servers instead, but I haven't ran into any issues with VZ so far). What's the difference between Docker and normal virtualization technology (OpenVZ/KVM)? Are there any good examples of when and where to use Docker over something like OpenVZ?