A virtual instrument is a software program or plug-in that uses a computer to produce sound by digital means. Virtual keyboards are one type of virtual instrument. Most virtual keyboards receive input signals via MIDI (Musical Instrument Digital Interface). When a virtual keyboard receives a MIDI signal, it either generates a tone in real time or triggers a prerecorded keyboard sample. MIDI signals can come from an external controller keyboard, from a MIDI sequence containing musical instructions, or from a mouse-click or computer keyboard input.
Musicians often choose to purchase virtual keyboards for the sake of convenience. Gigging musicians can carry a virtual keyboard installed on a laptop in a shoulder bag. Compare that to trying to move an upright piano from a rehearsal space to a second-floor venue. Virtual keyboards are far more portable and even easier to store. Software instruments don't need protective cases or cleaning and can't be damaged by humidity.
Musicians who can't afford a Bosendorfer piano or a Fender Rhodes Mark I might have the funds to purchase a virtual emulation. Especially in the case of vintage instruments, the purchase cost is only the beginning. Owners have to pay for constant maintenance, regular tunings and potentially expensive repairs. Vintage instruments can be difficult to find and keep in perfect working order and may need out-of-production peripherals or rare replacement parts. Likewise, virtual keyboard don't require routine cleanings and protective cases, which can be an added expense. An additional advantage to virtual keyboards for gigging musicians is the low risk of theft.
Virtual keyboards come in a variety of open-source and proprietary formats, many of which are cross-platform. These include Steinberg's Virtual Studio Technology (VST), Apple's Audio Units (AU), Digidesign's Real-Time AudioSuite (RTAS) and Linux's Virtual Studio Plugin Technology 2 (LV2).
Certain virtual keyboards are programmed with advanced features that make them more versatile than their hardware counterparts, including increased MIDI capabilities, polyphony and built-in effects.