AU
AU -Audio Units is the plug-in format developed by Apple to coincide with their new audio and MIDI technologies in OS X. Certain programs like Logic support Audio units exclusively, while other programs like Digital Performer 4 support their own MAS format as well as Audio Units. Audio Units are handled at the level of the OS X operating sytem itself and for programmers they can have a certain advantages in that two seperate engines exist for the GUI of the plug-in as well as the sound engine itself, allowing for a more advance user interface design.

DirectX
Direct X Plug-Ins Direct X is a real-time plug-in (i.e., Direct X plug-ins can process or alter the sound without creating a new file). Direct X uses "native processing" to function. Native processing uses the existing power of your CPU to process audio. Native processing has become more popular as CPU's keep getting faster. Direct X was created by Microsoft for Windows users to play fast video games without using DOS. One reason Microsoft created Direct X was that there were problems with hardware developers creating 2D and 3D accelerators. Microsoft saw an opportunity to unify the gaming industry, their result was Direct X. Direct X uses low level application programming interfaces (API's) for high performance multimedia applications such as plug-ins. It includes support for sound, music, graphics and network applications. Available on Windows system that uses Direct X, Windows 95, 98, 2000. Host programs must implement Direct X to run plug-ins.

HTDM
HTDM stands for "Host TDM" and refers to Plug-Ins that do all audio processing on the host CPU instead of the DSP chips found on your TDM hardware. They use a single, shared DSP on the TDM system to allow the audio from these host Plug-In processes to stream in or out of the system. The faster your computer, the more Plug-Ins you will be able to run. HTDM Plug-Ins allow the power and flexibility of host-based processing for synths and sampling to directly integrate into Pro Tools. You can work with these Plug-Ins as you would use standard TDM Plug-Ins, including major benefits like using Pro Tools automation (in addition to MIDI) to automate parameters, or using control surfaces for hands-on control of Plug-In parameters.

MAS
MAS (MOTU Audio System) is a real-time native plug-in format created by Mark of the Unicorn. MAS was created as a proprietary plug-in format for Performer and Digital Performer. MAS plug-ins are fully automatable and do not require external DSP. MAS is becoming a widely supported format by third party developers and currently supports a wide range of software synthesizers. Available on Macintosh OS systems that use MAS. Host programs must implement MAS to run plug-ins.

Others
Premiere - Premiere is a non real-time native plug-in format created by Adobe. It has been a popular format for quite some time. It is only available for Mac and is suppoted by a few pieces of host software like BIAS Peak, BIAS Deck, Emagic Logic Audio, Opcode's Studio Vision Pro. Available on Macintosh OS system that uses MAS. Host programs must implement Premiere to run plug-ins. AudioSuite - AudioSuite is a non-real-time plug-in format that provides file-based processing, meaning they process or alter the sound file and create a new file with the processed sound. The resulting effect is uniform across the entire file. AudioSuite Plug-Ins are great for conserving DSP power and certain types of processing where there is no real-time benefit or application, such as normalization and noise reduction. Available on all Pro Tools or ProTools LE systems on Mac OS, Windows 98, and Windows NT.

RTAS
RTAS (Real Time Audio Suite) is a new plug-in format created by Digidesign for Pro Tools LE. RTAS is the next generation of AudioSuite plug-ins. RTAS uses host based processing, your CPU, to run. RTAS is only limited by the available CPU processing power. RTAS offers similar performance to TDM in that each plug-in is fully automatable. Available on Digi ToolBox and Digi 001 (any system with Pro Tools LE) Mac OS & Windows 98.

TDM
TDM (Time Domain Multiplex) is a plug-in format created by Digidesign for Pro Tools systems. TDM can only function with Digidesign Farm cards. The number of plug-ins that you use depends on how many Farm cards you have and the type of plug-in. TDM is a 24 bit, 256 channel path that integrates mixing and real time digital signal processing. This plug-in format offers zero latency and is fully automatable. TDM is available for Pro Tools|24, Pro Tools|24 MIX and Pro Tools|24 MIXplus Mac OS & Windows NT. TDM plug-ins can also be used with TDM capable host programs like Logic Audio and Digital Performer when Farm Cards are used.

VST
VST (Virtual Studio Technology) is a real-time native plug-in format created by Steinberg. Currently there are over 300 plug-ins made for VST. It is becoming one of the most widely-used formats for audio plug-ins. VST plug-ins differ from Direct X plug-ins in a few areas: VST can be used on PC's or Mac's, it is dependent on the host program.