The term has already been mentioned several times, so it is time to ask: what is a container? There is a simple answer, a container in our mind is an instance of a Docker image. A Docker image is an ordered collection of root filesystem changes and the corresponding execution parameters.

When you scale your Azure Container Service, you typically run multiple containers from the same image.

A container is defined not only by a Docker image, but also by the following components:

• A so-called Dockerfile is a text file that contains necessary information about the structure of the container
• Something singular like a process, a service, or microservice (for example, an Azure Service Fabric) or an app

Containers take up less space than VMs (typically in a low MB range), and start almost instantly.

Now we have gained our first insights into the container issue. To understand the following, however, we must clarify a few more terms.