Many plug-ins have the option of being run as a stand-alone as well as through a VST host. Usually with the stand-alone version of a plug-in, settings can be made, some editing can be done, and it can be played via MIDI which can be easily set up along with the audio output within the plug-in itself. However, a VST host program like Cubase, controlling the plug-in by way of a MIDI track, is needed to record the sequence of notes that are played. Usually this is the best way to use any plug-in to its full potential, as many parameters can then also be automated and played back to modulate the resulting sounds.

With a stand-alone plug-in, MIDI data is routed directly to the instrument and it generates sound which is routed directly to the outputs of your audio interface that you have selected. This is fairly straightforward and probably easy enough to understand, or at least set up, though many people don't understand the signal path of a plug-in when it has been opened from within a VST host, and understanding this can make the overall use of the plug-in much more rewarding. The following is a simple explanation of this path.

When opened in a host like Cubase, a synth or sampler plug-in automatically creates a dedicated audio track or output channel, which allows you to control the volume or insert effects and more, depending on the capabilities of your host program. At this point you need a way to control the instrument, so you must create a MIDI track and attach its output to the plug-in.

Now whenever you send MIDI data to this track it triggers the plug-in and the plug-in creates an audio signal which you hear after it passes through its audio channel. At this point you may insert an effect into that audio channel in order to change or filter the sound in the way desired. Now that you have the plug-in feeding out audio, it can be easily recorded or mixed down to an audio track and then edited as a waveform.