[richiebee]

VST = Virtual Studio Technology. Invented by Steinberg (or at least, initiated by them). It really applies to a kind of Virtual Instrument that is supported by Steinberg DAW's (Cubase, Nuendo) as well as those of some other manufacturers. It's the VI format with the largest following and the widest support.

A VI in this case can be a software instrument (Sampler, synthesizer etc), or a software effect (compressor, reverb etc). There are also that are basically utilities, but they are applied either as an instrument or effect.