What is a container, and what does AWS ECS do?

A container is a way of running an application together with the files and dependencies it needs in an isolated environment. It helps reduce “it works on my machine” differences. It isn't the same as a full virtual machine and doesn't automatically give arbitrarily strong security isolation.

ECS is an AWS service that manages running such applications: which version to run, how many copies to keep and what to do when one stops working. For example, a Node backend can run as several containers. ECS itself doesn't replace the API code or the database. The configuration describes the application image, resources and permissions, among other things.