What is an image?

An image is a template, a "snapshot" of a future container. It stores everything needed to run an application: files, dependencies, configurations, library versions, build instructions.

Characteristics of an image:

1. Immutable - an image cannot be changed "on the fly"; it can only be rebuilt.
2. Layered - it consists of layers, which makes building and storage fast and economical.
3. Container template - any number of containers can be started from one image.
4. Portable - it works the same on any machine that has Docker.

In one line: an image is the template Docker uses to create a container.