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:<p><pre><code> What Is Docker? </code></pre> Good, I thought, finally an explanation. Then it says:<p><pre><code> So then what is Docker good at? </code></pre> Oh. That doesn&#x27;t help. Then:<p><pre><code> * 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 ... </code></pre> None of this tells me what docker actually is.<p>It goes on:<p><pre><code> Docker is a basic tool ... that you should start incorporating into your daily development and ops practices. </code></pre> Why? What is it?<p><pre><code> * Use Docker as version control system </code></pre> Why not just use git?<p><pre><code> * Use Docker when you want to distribute&#x2F;collaborate on your app&#x27;s operating system with a team </code></pre> Why not just use git?<p><pre><code> * Use Docker to run your code on your laptop in the same environment as you have on your server ... </code></pre> I already do that - why should I use docker?<p><pre><code> * Use Docker whenever your app needs to go through multiple phases of development </code></pre> Why?<p><pre><code> * Use Docker with your Chef Cookbooks and Puppet Manifests </code></pre> Why?<p><pre><code> * Docker doesn&#x27;t do configuration management </code></pre> What?<p>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.