OK, but what IS a plug-in?
The term plug-in describes a program that 'connects' to an host application's plug-in interface. Usually this is for the purpose of extending that application's functionality in some manner.Was the above a little unhelpful? OK, let's say you and a friend have formed a band. Between you both, you are responsible for writing songs, arranging, recording, and mixing. You need to be vocalists, drummers, guitarists, and keyboardists, and you need to make your own promotional material as well. In addition to this, there are some things you simply don't know how to do, for example you may be unsure how to take care of legal issues such as copyright. The obvious solution to this problem is to add members to your band who are better skilled at filling certain roles. You are looking, in effect, to 'plug in' some extra features or skills into your band. In effect this is what a software plug-in does. It adds a new 'skill' to an application. In terms of audio plug-ins, normally the new function (skill) would be some kind of audio processing, or generating. Examples of audio processing might be reverb or delay. For sound generating plug-ins, typically this would be as some form of synthesis. In short then, plug-ins can allow you to add new audio effects, and synthesis models to your favourite music applications.
Types of Plug-ins
In general there are three main types of plug-ins available for the mainstream music market. A few others exist, but these are typically proprietary, or specific to individual applications. For the remainder of this article, assume that 'plug-in' refers only to one of the 'standard' plug-in types.The first, and most common, type of plug-in adheres to Steinberg's VST specification and is available for both Mac and PC. With the support of Steinberg's own Cubase/Wavelab/Nuendo products, and a wide acceptance in the majority of audio applications, VST plug-ins have established themselves as the main player in town. In addition, a thriving development community has grown up around this standard, and as a result a huge range of quality plug-ins are available, sometimes even for free!
For PC users, the next main plug-in type is the DX, or DirectX, plug-in. This standard was developed by Twelve Tone Systems, Inc for their Cakewalk family of products. DirectX plug-ins are also well support in general by most major audio applications, but the standard has failed to attain the same critical mass that the VST specification has achieved. This is probably due in some large part to the fact that the DirectX standard does not exist on Macs, and developers looking for a cross-platform option tend to opt for VST.
Mac users now have a plug-in format of their own as well. With the advent of OS X, Apple added support for Audio Units, or AU plug-ins. It is too early to say for sure how well this new format will fare against the well established VST, but it is safe to assume that with the muscle of Logic Audio, they will quickly become an important part of any Mac musicians' lexicon.