A container is a lightweight, stand-alone, and executable software package that includes everything needed to run a piece of software, including the code, runtime, system tools, libraries, and settings. 

A container provides a lightweight runtime environment consisting of the core features of VMs and the isolated services of OSes. This makes the packaging and execution of microservices easy and smooth.

As the following diagram shows, a container runs as an application (microservice) within the OS. The OS sits on top of the hardware and each OS could have multiple containers, with one container running the application.

A container makes use of an OS' kernel interfaces, such as cnames and namespaces, which allow multiple containers to share the same kernel while running in complete isolation of one another. This gives the advantage of not having to complete an OS installation for each usage; the result is that the overhead is removed. This also makes optimal use of the hardware:

A container is a unit of software that packages code and its dependencies so the application runs quickly and reliably across computing environments.

https://cloud.google.com/learn/what-are-containers

Even though Docker is used as a synonym for containers, the reality is that they have existed long before Docker was a thing. Unix and Linux have had containers in some form or another since the late 70s, when chroot was introduced. Chroot allowed system admins to run programs in a kind-but-not-really-isolated filesystem. Later, the idea was refined and enhanced into container engines such as FreeBSD Jails, OpenVZ, or Linux Containers (LXC).

But what are containers?

A container is a logical partition where we can run applications isolated from the rest of the system. Each application gets its own private network and a virtual filesystem that is not shared with other containers or the host.

Running containerized applications is a lot more convenient than installing and configuring software. For one thing, containers are portable; we can build in one server with the confidence that it will work in any server. Another advantage is that we can run multiple copies of the same program simultaneously without conflict or overlap, something really hard to do otherwise.

However, for all this to work, we need a container runtime, a piece of software capable of running containers.

https://www.docker.com/resources/what-container/