A “Virtual Appliance” is a virtual machine (or “VM”) that has already had an Operating System installed and configured for a particular purpose. For example, a VM may be set up to be an E-Mail server, or a DNS server, or a Web server, or a desktop environment for end user use.
VMWare, the leading VM software maker has multiple, pre-made “Virtual Appliances” that you can download, for free, from their website. These are usually created and configured by their customers and shared with the community. They’re an enormous time saver if you need a server and don’t have the time or the know-how to set one up from scratch. Just download a pre-made one and go!