Virtual appliance

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A virtual appliance is a minimalist virtual machine image designed to run under VMware, Xen, Microsoft Virtual PC, QEMU, Usermode Linux, CoLinux or other PC virtualization technology, providing network applications like firewalls or webservers.

Virtual appliances are a subset of the broader class of software appliances. Like software appliances, virtual appliances are aimed to eliminate the installation, configuration and maintenance costs associated with running complex stacks of software.

A key concept that differentiates a virtual appliance from a virtual machine is that a virtual appliance is a fully pre-installed and pre-configured application and operating system environment whereas a virtual machine is, by itself, without application software.

Typically a virtual appliance will have a web interface to configure the inner workings of the appliance. A virtual appliance is usually built to host a single application, and so represents a new way of deploying network applications.

As an example, the MediaWiki software that powers Wikipedia is available as a virtual appliance. This appliance contains all the necessary software, including operating system, database and MediaWiki, to run a wiki installation as a "black box".