VST ( Virtual Studio Technology ) was created by Steinberg to make a complete software studio possible. This technology allows third-party software developers to create real-time effect, metering and instrument modules that can "plug in" to a Digital Audio Workstation (DAW).

The only piece of software you will need is a VST compatible host application (DAW, audio editor softwares, vst racks...). There are many of these available, here's a list of free VST hosts. You will also need a fairly powerful CPU, since VST instruments and effects work in real time, and in some cases can be quite draining on your computers processing power. An ASIO driver compatible soundcard is also required to play or process sound in real-time.

Generally speaking there are 3 types of VST plug-ins available:
VSTi (Instruments) - Various devices that create sound, be it through synthesis or through sampling (or both).
VSTfx (Effects) - Various devices which process sound, a few examples are reverberation, delay, phasers and distortion units...
VSTmidi - MIDI tracks plug-ins which manipulate or generate MIDI data (i.e. arpeggiators, controllers, chorders...).

VSTs can be some of the easiest programs to install, since they don't have to be able to run by themselves. Usually all you have to do is drag the DLL file into your plug-ins folder, and the next time you fire up your DAW, the new plug-in should appear in your list of available VSTs or VSTIs. With the ones that comes as a EXE file, you will need to trigger the setup file in order to install them in your DAW plug-ins folder.