Docker is a platform designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all parts it needs, such as libraries and other dependencies, and ship it all out as one package. By doing so, the developer can be assured that the application will run on any other Linux machine regardless of any customized settings that machine might have that could differ from the machine used for writing and testing the code.

1. Containers:

   • Lightweight and portable encapsulations of an environment in which to run applications.
   • Share the host system's kernel but run in isolated processes.

2. Images:

   • Read-only templates used to create containers.
   • Can be built from a Dockerfile or pulled from a Docker registry like Docker Hub.

3. Docker Engine:

   • The core part of Docker, responsible for creating and managing containers.

4. Docker Hub:

   • A cloud-based registry service which allows you to link to code repositories, build your images, and test them.

• Consistency: Ensures that the software will run the same way, regardless of where it is deployed.
• Isolation: Each container runs in its own isolated environment, which helps in avoiding conflicts.
• Portability: Containers can run on any system that supports Docker, making it easy to move applications between environments.
• Efficiency: Containers are lightweight and use fewer resources compared to traditional virtual machines.

Let's look at a simple example to understand how Docker works. We'll run a basic web server using Docker.

1. Pull an Image:

   • We'll use the official Nginx image from Docker Hub.

   docker pull nginx
   

2. Run a Container:

   • Create and run a container from the Nginx image.

   docker run --name my-nginx -d -p 8080:80 nginx
   

   • --name my-nginx: Names the container "my-nginx".
   • -d: Runs the container in detached mode (in the background).
   • -p 8080:80: Maps port 8080 on the host to port 80 in the container.

3. Access the Web Server:

   • Open a web browser and navigate to http://localhost:8080. You should see the Nginx welcome page.

• Pulling an Image: The docker pull command fetches the specified image from Docker Hub.
• Running a Container: The docker run command creates a new container from the specified image and starts it. The -p flag maps the container's port to the host's port, allowing you to access the service running inside the container.

In this section, we covered the basics of what Docker is and why it is useful. We discussed key concepts such as containers, images, Docker Engine, and Docker Hub. We also walked through a practical example of running a simple web server using Docker. This foundational knowledge will be built upon in subsequent modules as we dive deeper into Docker's capabilities and features.