Containerization is a form of operating-system-level virtualization or application-level virtualization over any kind of machine that can run the container runtime.
Containers are a standard unit of software that packages up code and all its dependencies, including the operating system, libraries, system, etc. into a computing environment that can quickly and reliably run on any machine. Container images tend to be lightweight and portable, and can be used to deploy applications on any machine under any configurable environment that native deployments can configure.
Container images become containers at runtime and in the case of container runtimes - images become containers when they are executed. Regardless of whether the runtime runs on Linux, Unix, Windows, or macOS, the container is always executed in the same way no matter the infrastructure. Containers are isolated from one another and bundle their own resources, so the applications run uniformly and their isolation provides some greater degree of separation or sanboxing from the host OS actually running the container. This provides these benefits.
Docker is a software stack that provides containerization of applications and virtualized operating systems. It was one of the first containerization platforms, that gained prominence in the early 2010s.
Docker in the 2020s is a mature platform that is used by many organizations. Although the landscape of containerization platforms has changed significantly, Docker is still a popular choice for many organizations despite the rise of other containerization platforms that address some of the limitations of Docker.