What is the difference between a Docker container and a virtual machine?

Answer

A virtual machine (VM) virtualizes hardware and runs a full guest operating system on top of a hypervisor, so each VM carries its own kernel and is heavy in size and boot time. A container virtualizes the operating system instead, sharing the host kernel and isolating only the application and its dependencies, which makes it megabytes rather than gigabytes and starts in seconds. VMs offer stronger isolation and can run different operating systems, while containers offer higher density and faster deployment. Many production systems combine both, running containers inside VMs for layered isolation.