Key Points

Containers were popularised by Docker in the 2010s, following milestone technologies such as GUIs, web browsers, and hypervisors.

A container is a lightweight package of software — containing the application, libraries, and all dependencies needed to run it. It is not a full operating system.

Containers are portable:Any system with a Linux kernel and a container runtime can run them, eliminating the 'it works on my machine' problem.

Containers are efficient: Small in size, they start and stop in seconds (sometimes under a second). They can be long-running (e.g. a web server) or short-lived (e.g. a one-off task).

Containers run in isolation: Each container is kept separate from others, even when sharing host OS resources. Different versions of the same app can run side by side.

Containers are scalable: With appropriate automation, containers can be spun up or torn down dynamically in response to demand.