VST (Virtual Studio Technology) is a virtual studio technology. Today it is not just an industry standard, but also the most popular application plug-in format with which modern composers, DJs and arrangers create their music.
The VST format was developed in collaboration between Steinberg and Propellerhead. VST plugins are completely tied to the capabilities of the computer hardware. These apps are designed to connect to music programs. Plugins work in real time. Sound processing is done using processor resources. The more powerful the processor of the computer, the better and more stable the program will work there.
There are several versions of Virtual Studio Technology. The latest and most recent version is VST3. The developers took into account the accumulated experience. It replaced the previous versions, but at the same time the plugins of the previous versions will be able to work well within the music program.
The AU (Audio Units) format was developed by Apple shortly after the introduction of VST2. The format is a set of APIs and other instructions that allow macOS and iOS operating systems to generate, process, play and manipulate audio with minimal latency. This format is provided for all gadgets produced by the Cupertino-based company. The point is that plugins of different formats do not work in the same way on different operating systems. For example, VST can work on both Windows and iOS, but in the first case, the operating system is much more comfortable for it.
Apart from the ability to work both as virtual instruments and musical effects, there are no fundamental differences between AU and VST. In fact, the AU plugin format is an iteration of the VST modified for better compatibility with macOS. The similarity to VST allows developers to convert VST plugins to AU plugins for easy porting of products to the macOS environment.
With the development of smartphones and tablets, and based on user feedback, Apple has introduced an improved version of the format – AUv3. The update is an extended set of APIs that are compatible with iOS and iPadOS. Developers can create versatile plug-ins for iPhone, iPad, and Mac, and users get a suite of plug-ins that are compatible with both desktop and mobile DAWs (such as Garage Band or Cubasis).
So, the AU format is more acceptable for those musicians who use Apple technology. There are no cardinal differences in this plugin format. But using it on computers with the Windows operating system is simply unproductive.