Basic docker-related vocabulary (namely Image, Container and Volume) with a brief explanation. There are a lot of nuances I’ll be glossing over.
We’ll go into a bit more practical detail in the follow-up post.
Not a VM
It is usually described in detail how different Docker (or containerization more broadly) is a from a traditional Virtual Machine.
This is most definitely true: there are a number of low level and practical differences between these two technologies. However, I would argue that pointing out those differences does little to help understand the vocabulary and concepts around Docker.
In fact, understanding VM-related concepts like instance or virtual hard disks and the difference between a VM definition/configuration and a specific run of that VM can go a long way to help understand why docker can be so intricate and useful.
Image
An Image is what defines the default composition and behavior of a Container. Think of how the idea of a table is an abstract representation of a concrete, palpable table.
If we were talking about a VM, this would be more or less analogous to a VM’s definition or configuration.
You define an Image in the corresponding Dockerfile, and build it via the command line (or pull it from the web).
In a VM, configuration and behavior would usually go separately. Best you could do are snapshots. An Image on the other hand, not only defines configuration (i.e. OS or installed software) but also behavior (i.e. commands to run, dependencies to install) of what we call Containers.
Container
A Container is a concrete instantiation of what’s defined by the corresponding Image. Think of the palpable table from before.
This would be like a specific, concrete instance of your good old VM.
It’s an isolated environment in which ‘stuff’ (might be your app, might be other things) happens. You can think of a Container as the concrete instantiation of an Image It gets created when you run said Image.
The technical differences between VMs and containerization are usually brought up at this point. Just know that Containers are stupidly efficient compared to VMs, and a lot more versatile.
Note
Technically, we say that Images run in Containers.
So Containers only hold that Image run. This is because one Image can be executed multiple times in parallel, so you might have a bunch of Containers running with the same Image but possibly with different processes and/or outputs or results.
However, I reckon it’s easier to visualize for a newcomer as explained above.
Volume
Think of it as ‘disk space’ for a Container (or multiple Containers). It’s where docker will operate, its very own file system.
Plain and simple, it’s the equivalent of a virtual hard disk for a VM. It can be, and usually is, shared between multiple Containers and can easily communicate with (as in it’s mounted to) the hosts file system.