What is docker, when to use it (opens in new tab)

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.