VST

VST plugins generally run within a DAW to provide additional functionality. Most VST plugins are either instruments (VSTi) or effects, although other categories exist—for example meters or analyzers. VST addons generally provide their own GUI (graphical user interface) to display knobs, keyboard or other features onscreen.

VSTi can simulate original equipment, so a software copy of a keyboard/synthesizer or sampler. This way you may can purchase the instrument you always wanted to have but had no space to store it or not enough cash to buy it.

Generally the usage of a VST makes your life easier and speeds up lenghty development and patching process within your DAW.

The following website provide a good overview of top VST:
Music Radar
Plugin Boutique

The below VST I have purchased (if not freeware) and use frequently in my work:

There are many many more out there but again, no time to see it all…

After installing your favourite VST to your computer your DAW will automatically recognise the availability of your new plugin and you can use it in your daily operation.