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Docker Explained: History Before Containerization
Before containerization came into the picture, the leading way to isolate, organize applications and their dependencies was to place each and every application in its own virtual machine. These machines run multiple applications on the same physical hardware, and this process is nothing but Virtualization.

But virtualization had few drawbacks such as the virtual machines were bulky in size, running multiple virtual machines lead to unstable performance, boot up process would usually take a long time and VM’s would not solve the problems like portability, software updates, or continuous integration and continuous delivery.

These drawbacks led to the emergence of a new technique called Containerization. Now let me tell you about Containerization.

Containerization
Containerization is a type of Virtualization which brings virtualization to the operating system level. While Virtualization brings abstraction to the hardware, Containerization brings abstraction to the operating system.
Docker Explained: Reasons to use Containers
Following are the reasons to use containers:

Containers have no guest OS and use the host’s operating system. So, they share relevant libraries & resources as and when needed.
Processing and execution of applications are very fast since applications specific binaries and libraries of containers run on the host kernel.
Booting up a container takes only a fraction of a second, and also containers are lightweight and faster than Virtual Machines.
Docker Explained: What is Docker?
Docker is a platform which packages an application and all its dependencies together in the form of containers. This containerization aspect of Docker ensures that the application works in any environment.

Containerization - Docker Explained As you can see in the diagram, each and every application runs on separate containers and has its own set of dependencies & libraries. This makes sure that each application is independent of other applications, giving developers surety that they can build applications that will not interfere with one another.

So a developer can build a container having different applications installed on it and give it to the QA team. Then the QA team would only need to run the container to replicate the developer’s environment.

Dockerfile: A Dockerfile is a text document which contains all the commands that a user can call on the command line to assemble an image. So, Docker can build images automatically by reading the instructions from a Dockerfile. You can use docker build to create an automated build to execute several command-line instructions in succession.

Docker Image: In layman terms, Docker Image can be compared to a template which is used to create Docker Containers. So, these read-only templates are the building blocks of a Docker Container. You can use docker run to run the image and create a container.

Docker Images are stored in the Docker Registry. It can be either a user’s local repository or a public repository like a Docker Hub which allows multiple users to collaborate in building an application.

Docker Container: Docker Container is a running instance of a Docker Image as they hold the entire package needed to run the application. So, these are basically the ready applications created from Docker Images which is the ultimate utility of Docker.

Docker Explained: Docker Compose & Docker Swarm
Docker Compose is a YAML file which contains details about the services, networks, and volumes for setting up the Docker application. So, you can use Docker Compose to create separate containers, host them and get them to communicate with each other. Each container will expose a port for communicating with other containers.

Docker Swarm is a technique to create and maintain a cluster of Docker Engines. The Docker engines can be hosted on different nodes, and these nodes, which are in remote locations, form a Cluster when connected in Swarm mode.