A virtual appliance is a minimalist virtual machine image designed to run under some sort of virtualization technology (like VMware Workstation, Citrix XenServer, VirtualBox or many others).

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 Spam filter software that powers antispam gateway server is available as a virtual appliance. This appliance contains all the necessary software, including operating system, database and mail server, to run preconfigured installation as a "black box".