A virtual machine can exist as a standalone machine or it can exist within a vApp.
A virtual machine is a software computer that, like a physical computer, runs an operating system and applications. The virtual machine consists of a set of specification and configuration files, and is backed by the physical resources of a host. Every virtual machine has virtual devices that provide the same functionality as physical hardware but are more portable, more secure, and easier to manage. Virtual machines can be standalone, or they can exist within a vApp. A vApp is a compound object composed of one or more virtual machines, as well as one or more networks.