What distinguishes a Docker container from a virtual machine?

A Docker container is distinct from a virtual machine primarily because it shares the host operating system kernel. This means that multiple containers can run on a single host operating system instance without the need for a separate OS for each one. This sharing of the kernel allows containers to be lightweight and to start up quickly compared to virtual machines, which require a full guest OS stack, including a hypervisor layer, for each instance.

This architecture enables more efficient use of system resources, as the overhead associated with running multiple separate operating systems is eliminated in a containerized environment. Additionally, since containers can utilize the host's kernel, they can also achieve high levels of performance and scalability.

This characteristic is fundamentally what differentiates Docker containers from traditional virtual machines, which encapsulate entire operating systems and are thus more resource-intensive and slower to start.