If you're working inside a digital audio workstation (DAW) and you want some nice reverb or maybe a compressor or two, you're going to need a plug-in. A plug-in is the software equivalent of a hardware effects processor or hardware instrument. Depending on what recording software you opt to run, you might need different types of plug-ins. Each recording program requires plug-ins to be written in a specific language that the host program will understand. Luckily, most plug-in manufacturers include multiple versions. Let's break down all the major formats so you can see what's around.
Virtual Studio Technology
Virtual studio technology (VST) is a plug-in standard format created by Steinberg, which also makes Cubase and Nuendo recording software. VST plug-ins work in Cubase, and other programs have adopted the use of VST plug-ins as well. VST plug-ins are available for both Macs and PCs. Because VST has been around for a long time, there is a nice selection of VST plug-ins available.
Real-Time Audio Suite
Real-time audio suite (RTAS) is the only plug-in format that works inside Pro Tools. Because of the widespread use of Pro Tools systems, there are many RTAS plug-ins available on the market. Because Pro Tools runs on PCs and Macs, you can find RTAS for both. The British company FXpansion has created a VST-to-RTAS converter allowing the many VST plug-ins to be utilized inside Pro Tools.
DirectX is a Windows-only plug-in format that is supported by FL Studio and a few other programs. DirectX was originally a multimedia language introduced by Microsoft to write games and other multimedia programs. Over the years, DirectX has fallen out of favor as the VST format now dominates Windows applications.
Audio Unit is a plug-in format introduced by Apple as part of its remake of the Mac operating system version 10 (also known as OS X). The thought behind Audio Units was to create a system-level plug-in format that worked inside the operating system and could be available to any audio program on the system. Logic, Digital Performer, and Live all support Audio Units plug-ins. Apple's free recording software GarageBand utilizes Audio Unit plug-ins as well. Audio Unit plug-ins are available only on the Mac platform.
MOTU Audio System
MOTU audio system (MAS) is the plug-in standard written by MOTU for use in its Digital Performer application. Before Version 4 of Digital Performer, MAS was the only plug-in that could be used in Digital Performer; however, now Digital Performer uses MAS and Audio Units. Since Digital Performer is Mac only, no Windows versions of MAS plug-ins exist.
A wrapper is a program that converts plug-ins from one type to another. On the PC side, there are DirectX-to-VST wrappers that extend the choices of Windows plug-ins. FL Studio can act as a DirectX-to-VST converter. On the Mac side, there is a VST-to-Audio Unit wrapper that greatly improved the number of available plug-ins while OSX was just starting out, and it continues to be updated to this day.