VST audio plug-in

Virtual Studio Technology (VST) is a software interface that integrates software audio synthesizer and effect plugins with audio editors and recording systems. VST and similar technologies use digital signal processing to simulate traditional recording studio hardware in software. Thousands of plugins exist, both commercial and freeware, and a large number of audio applications support VST under license from its creator, Steinberg.

VST plugins generally run within a digital audio workstation (DAW), to provide additional functionality, though a few standalone plugin hosts exist which support VST. Most VST plugins are either instruments (VSTi) or effects (VSTfx), although other categories exist—for example spectrum analyzers and various meters. VST plugins usually provide a custom graphical user interface that displays controls similar to physical switches and knobs on audio hardware. Some (often older) plugins rely on the host application for their user interface.

VST instruments include software simulation emulations of well-known hardware synthesizers and samplers. These typically emulate the look of the original equipment as well as its sonic characteristics. This lets musicians and recording engineers use virtual versions of devices that otherwise might be difficult and expensive to obtain.

VST instruments receive notes as digital information via MIDI, and output digital audio. Effect plugins receive digital audio and process it through to their outputs. (Some effect plugins also accept MIDI input—for example MIDI sync to modulate the effect in sync with the tempo). MIDI messages can control both instrument and effect plugin parameters. Most host applications can route the audio output from one VST to the audio input of another VST (chaining). For example, output of a VST synthesizer can be sent through a VST reverb effect.

...
Wikipedia