Docker 101
Docker [1] manages and runs containers, a thing that acts like an operating system. It is similar to a virtual machine, but a container uses a lot of the underlying operating system (called the "host") to work. Instead of building a whole operating system with emulated hardware, its own kernel, and so on and so forth, a container uses everything it can from the underlying machine, and, if it is well-designed, implements only the bare essentials to run the application or service you want it to run.
Whereas virtual machines are designed to run everything a regular machine can run, containers are usually designed to run very specific jobs. That is why Docker is so popular for online platforms: You can have a blogging system in one container, a user forum in another, a store in another, and the database engine they all use in the background in another. Every container is perfectly isolated from the others. Docker allows you to link them up and pass information between them. If one goes down, the rest continue working; when the time comes to migrate to a new host, you just have to copy over the containers.
But there's more: Docker is building a library of images [2] that lets you enjoy whole services just by downloading and running them. These libraries are provided by the Docker company or shared by users and go from the very, very general, like a WordPress container [3], to the very, very niche, like a container that provides the framework to run a Minetest [4] server [5].
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