A virtual machine, also commonly referred to as VM, is a guest system running on top of a virtualization software or hypervisor. VirtualBox, VMWare, and QEMU are examples of popular tools that are able to emulate network, disk, and other hardware resources to build virtualized environments that behave as physical computers. These environments are isolated from each other and from the host where the virtualization software is installed, each running distinct operating systems.
Virtual machines are largely used in cloud computing to distribute hardware resources among smaller-sized, virtual private servers. When compared to containers, virtual machines are more resource-intensive, but that allows them to emulate entire servers and desktops seamlessly.
Another important use case for virtual machines is within the context of development environments. With virtual machines, developers are able to work on their applications in isolated, pre-configured environments that are independent of their base operating system.