A container is a lightweight, standalone, and executable software package that includes everything needed to run a piece of software. And one of the most popular tools for working with containers is Docker. Docker is both the name of the company (Docker Inc) and the software they have created which

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Containers provide a portable and efficient way to package applications and their dependencies, ensuring consistency across various environments. The benefits they bring to software development is similar to the benefits brought to the global economy by the humble shipping container.

How Docker Containers Work – Explained for Beginners

Great overview article that uses diagrams to efficiently explain docker.
TLDR:
Here is a condensed 4 point summary:
<ol>
<li>Containers provide portable virtual environments for apps.</li>
<li>Docker builds images from Dockerfiles and runs containers from images.</li>
<li>Containers are efficient by sharing the OS and virtualizing only apps.</li>
<li>Standard formats ensure portable containers like standardized shipping containers.</li>
</ol>

Good analogy with real containers