"Desktop virtualisation" isn't a specific technology or even a single delivery method. It's a broad description that includes all the ways it's possible to use a desktop, laptop or other device to access data or applications that live somewhere else.

Usually that means a user with a PC interacting with an application that's running on a server in the data centre. But there are a lot of ways to make that happen. According to a survey conducted by Enterprise Management Associates, most companies that implement virtual desktops do so using several different delivery methods.

Virtual applications

  • Web application: Browser interface for an application running on a server. Not what you usually think of as "desktop virtualisation," web apps do fit the definition and are tied for the most common method of delivery.
  • Remote viewing: Typically described as "application virtualisation," this method allows users to view and control an app from their desktops, though the application itself runs on a backend server. Most often a user logs into an app that many use at once. Less often, a user launches a separate instance of the application for herself.
  • Streaming application: The application lives on a network server, and when a user launches it, portions of the application stream down to the user's PC to be executed locally. Takes advantage of PC-based memory and processing power, but puts greater load on the network than remote viewing.

Virtual desktop

  • Remote computing: Similar to remote-viewed applications, but the entire operating system runs on the server. The user's PC, which may be a dumb terminal with no processing capability of its own, serves only to send keystrokes and mouse clicks to the server. Users log in to a shared instance of the OS, which can support far more users per server than alternatives.
  • Virtual Desktop Infrastructure (VDI): This term is sometimes applied to shared desktop models, or streaming models, but more typically means that users log in to a server-based virtual machine (VM) running an operating system and applications accessible to only one user at a time. Applications that can't be used in shared systems are usually fine running in standalone VMs, but giving each end user a separate VM drastically cuts the number any given server can support.
  • Streaming desktop: Similar to streaming applications, when the user logs in, relevant portions of the operating system and applications download across the network into a virtual machine inside the user's PC. The OS and applications function there just as if they were on a standalone PC, but are isolated from the rest of the computer by the virtual machine.
  • Disconnected virtual desktop: As with a streaming desktop, the OS and applications execute within a virtual machine on the user's PC, but the VM includes caching capabilities that allow the user to operate while disconnected from the network. The VM keeps the OS and application isolated from the rest of the machine, and syncs data back to the network upon reconnection.
  • Mobile virtual desktop: Hypervisors designed for iPhones and other mobile devices can create virtual machines within which some OS and application code can execute. More commonly, they allow portable devices to become viewing platforms (similar to dumb terminals) for server-based virtual desktops or applications.

Mixing an matching methods

There has been a tremendous shift toward the virtualisation of some parts of the desktop.

By far the most popular, and unrecognised, method of desktop virtualisation is to put a web interface on a server-based internal application, a method more than 70% of companies surveyed by Enterprise Management Associates (EMA) have already used.

Enterprises tend to deploy four or five different endpoint virtualisation technologies on average, and almost a quarter deploy more than six different technologies simultaneously, explains Andi Mann, an EMA analyst. In addition to web-based applications, there are plenty of server-based applications without web frontends, not to mention applications or data accessible only from hidden servers, cloud computing service providers, or any commercial web application provider, including Google, Salesforce.com and Microsoft.

And that's without even starting on the "real" types of virtual desktop, which primarily involve applications streamed to a standalone PC, dumb terminal logins to a shared OS session, and standalone operating systems accessed by single users as their own virtual machines.

It may not be a great idea to mix and match all these delivery methods simultaneously, but they're available so IT people can give the end users exactly the kind of computing resources they need. If someone isn't demanding, maybe she gets the shared OS. If a user needs only one app, that can be streamed. If someone moves around a lot and needs to access a lot of graphics intensive applications from around the globe, maybe IT reserves a VM for him.