Docker is a complete containerization platform that provides an easy-to-use interface for building and running containers, while containerd is a lightweight container runtime that focuses solely on managing container lifecycles without additional tooling. Docker is positioned as an end-to-end developer-oriented container engine, including everything needed to use containers on a host, such as image management, storage, networking, and a CLI. Containerd, on the other hand, is a portable container runtime that provides a daemon for orchestrating lower-level container management functions, allowing higher-level tools like Docker to achieve key container tasks without reimplementing basic functionality themselves. The two projects are closely related but separate, with Docker focusing on providing a streamlined user experience and containerd implementing essential container runtime capabilities that other tools can use. Containerd is still a relatively heavyweight solution, internally calling a lower-level runtime component, runc, to actually start containers.