Glossary

Virtual Machine (VM)

A virtual machine is an emulation of a complete computer running on top of a physical host. A VM has its own OS kernel, its own disk, and its own network. A hypervisor (VMware, KVM, Hyper-V) distributes resources among multiple VMs on a single physical server.

VM vs Container

VM: hardware-level isolation, full OS (~GB), starts in minutes, maximum security
Container: process-level isolation, shared host kernel (~MB), starts in seconds, slightly less isolation

When VM, when container

VMs — when strong isolation is required (different OSes, compliance, untrusted code). Containers — for most microservice applications where startup speed and density matter.