What is Docker?

A tool for building and running applications in containers.

In plain English

Docker packages an application and what it needs to run into an image. A container is a running instance of that image. Because the package carries its own dependencies, it behaves the same on a laptop, a test server and in the cloud. Kubernetes can then run and manage many such containers.

Example

A Dockerfile lists the base image, copies the code in and says which command to run. Building it produces an image you can start anywhere.

Easy to confuse with

• Container vs virtual machine: Docker is a tool for containers. Container versus VM compares the two ways of isolating software.

Where you will meet it

This idea belongs to the subject area of these practice tests. We do not publish or predict real exam questions.

• KCNA practice tests
• GitHub GH-900 practice tests

Related terms

• Container vs virtual machine: A virtual machine includes a whole operating system. A container shares the host’s operating system kernel and packages only the app and what it needs.
• Pod: The smallest unit you can deploy in Kubernetes, made of one or more containers that run together.
• CI/CD: Automatically building, testing and delivering code every time it changes.

Written and checked by the Be Ready Education team, last reviewed 2 November 2026. General study information, not professional advice. Providers change names and rules, so confirm details on the official documentation. Not affiliated with, endorsed by or sponsored by any certification vendor or exam body. Certification names, exam codes and product names are trademarks of their respective owners and are used only to identify the exam a practice product prepares for. Our questions are original and are not actual exam questions.