What is docker, when to use it

This, like so many other articles in the technical world, actually assumes you know what docker is. For example, it says:

    What Is Docker?

Good, I thought, finally an explanation. Then it says:

    So then what is Docker good at?

Oh. That doesn't help. Then:

  * Docker is great at building and sharing disk images ...
  * Docker is a manager for infrastructure ... 
  * Docker is a great image distribution model ...
  * Docker uses btrfs ...
  * Docker has a central repository of disk images ...

None of this tells me what docker actually is.

It goes on:

    Docker is a basic tool ... that you
    should start incorporating into your
    daily development and ops practices.

Why? What is it?

  * Use Docker as version control system

Why not just use git?

  * Use Docker when you want to distribute/collaborate
       on your app's operating system with a team

Why not just use git?

  * Use Docker to run your code on your
       laptop in the same environment as
       you have on your server ...

I already do that - why should I use docker?

  * Use Docker whenever your app needs to go
       through multiple phases of development

Why?

  * Use Docker with your Chef Cookbooks and
       Puppet Manifests

Why?

  * Docker doesn't do configuration management

What?

At this point I give up - I have no idea what the author is talking about, and cannot deduce who the expected audience might be.