A virtual keyboard is a software and/or hardware component that allows a user to enter characters. A virtual keyboard can usually be operated with multiple input devices, which may include an actual keyboard, a computer mouse, a headmouse, and an eyemouse.

On a desktop PC, one purpose of a virtual keyboard is to provide an alternative mechanism for disabled users that cannot use a physical keyboard. Another major use for an on-screen keyboard is for bi- or multi-lingual users, who continually need to switch between different character sets and/or alphabets. Although hardware keyboards are available with dual layouts (for example Cyrillic/Latin letters in various national layouts), the on-screen keyboard provides an handy substitute while working at different stations or on laptops, which seldom come with dual layouts.

The standard on-screen keyboard utility on most Windows systems allows hot-key switching between layouts from the physical keyboard (typically alt-shift but this is user configurable), simultaneously changing both the hardware and the software keyboard layout. In addition, a symbol in the sys-tray alerts the user to the currently active layout. Although Linux supports this fast manual keyboard-layout switching function, most popular Linux on-screen keyboards such as gtkeyboard, Matchbox-keyboard or Kvkbd do not react correctly. Kvkbd for example defines its visible layout according to the first defined layout in Keyboard Preferences rather than the default layout, causing the application to output incorrect characters if the first layout on the list is not the default. Activating a hot-key layout switch will cause the application to change its output according to another keyboard layout, but the visible on-screen layout doesn't change, leaving the user blind as to which keyboard layout he is using. Until these deficiencies are corrected, Linux on-screen keyboards remain of limited usefulness for multi-lingual / multi-alphabet users.

On devices which lack a physical keyboard (such as personal digital assistants or touchscreen equipped cell phones), it is common for the user to input text by tapping a virtual keyboard built into the operating system of the device. Virtual keyboards are also used as features of emulation software for systems that have fewer buttons than a computer keyboard would have.

Virtual keyboards can be categorized by the following aspects:

• physical keyboards with distinct keys comprising electronically changeable displays integrated in the keypads
• virtual keyboards with touchscreen keyboard layouts or sensing areas
• optically projected keyboard layouts or similar arrangements of "keys" or sensing areas
• optically detected human hand and finger motions
• JavaScript virtual keyboards used to translate the input from one keyboard layout to another

An optical virtual keyboard has been invented and patented by IBM engineers in 1992. It optically detects and analyses human hand and finger motions and interprets them as operations on a physically non-existent input device like a surface having painted keys. In that way it allows to emulate unlimited types of manually operated input devices such as a mouse or keyboard. All mechanical input units can be replaced by such virtual devices, optimized for the current application and for the user's physiology maintaining speed, simplicity and unambiguity of manual data input.