When Steinberg (the Cubase people) introduced the VST plugin way back in the distant past, I'm not sure that they had any idea of what they were unleashing for the world of computer-based music. Before VST, when you purchased your PC software for recording you were locked into all of the features of your chosen audio workstation, for better or worse.
And then there was Virtual Studio Technology (VST)! The ability to add plugins at your discretion (or when finances permitted), in a format that was compatible with your Cubase software. Okay, there was that problem to begin with - you needed to be using Cubase.
The VST plugin format has become extremely popular, especially in Windows-based software, but is also used in cross-platform products, where OSX is supported.
Avid's Pro Tools introduced their own plugin format (RTAS and then, more recently, AAX), as did Logic when they went Apple Mac only with their Audio Unit (AU) format.
Some DAW software is clever enough to recognise VST and AU format plugins, including Presonus Studio One, Ableton Live and Bitwig Studio.
When Propellerhead entered the plugin market with an update of Reason a few years ago, they decided on their own format that would integrate with their already complex (but very stable) system, called RE (for Rack Extension). These plugins have to be able to patch back and forth with the basic functionality of Reason, which is something I really appreciate about this amazingly modular DAW. Devices and plugins can be patched into each other giving levels of control and audio flexibility not found in other DAWs.
When you are downloading and installing plugins on your PC, the installer will give you options for what you want to install, especially on Mac, VST/AU/AAX. Only install what you need for your system/DAW.
Several software companies have introduced shell programmes, called wrappers, that allow you to use VST plugins in AU format, and vice versa, giving access to more possibilities for your music making.
So, when you are purchasing plugins for your DAW, you need to read all of those boring ‘system requirements’ that tell you how powerful your computer needs to be (some plugins are 32-bit and are not compatible with newer 64-bit operating systems), what version of operating system is supported, and how much hard drive space is needed (especially important if you are installing sample player plugins that have a very large library included, for example, the Key Stats for Spitfire's Symphonic Strings).