Virtual or vst instruments / plug-ins are software applications that once installed enable the user to trigger sounds on their computer, usually with some kind of MIDI controller like a keyboard. Virtual instruments are most commonly used as plugins hosted inside sequencers or DAWs like Ableton, Cubase, FL Studio, Garageband, Logic and Pro Tools, or they can be used standalone. When used inside a sequencer, the user can record their virtual instrument peformances and turn them into arrangements or songs.
There are two main types of virtual instrument. The first includes a large core library of pre-recorded sounds that are triggered via a software interface similarly to a sampler. The second is algorithm based and works by creating and modulating waveforms in a similar way to a synthesizer. Some virtual instruments like Spectrasonics Omnisphere work by combining both of these methods together, giving the user the best of both worlds.