A virtual machine is a self-contained operating environment that behaves like a separate computer while running on a physical host. A VM includes its own guest operating system, virtual CPU, memory, storage, and network interfaces. Multiple VMs can run on a single physical server through a hypervisor, which allocates and manages physical resources. A hypervisor enables virtualization, but it is not the guest operating environment itself. A container packages an application and dependencies while sharing the host operating system kernel, making it lighter than a VM. A WAN accelerator improves performance over wide area links and is unrelated to virtualization. VMs are foundational to cloud computing because they allow providers to abstract physical hardware and offer flexible compute resources to customers. Security teams must secure VMs by hardening guest operating systems, patching, controlling access, monitoring activity, and applying cloud network policies. Reference/topics: Cloud Security 5.4, virtualization and virtual machine; Cloud Security 5.2, IaaS.